JAMAICA. Its take-charge leader is running a race between development and discontent. Edward Seaga has reversed Jamaica's economic slide and won more US aid and trade than any other Caribbean leader. President Reagan would like to make Jamaica a Caribbean showcase. But the results of the two leaders' efforts are far from decisive. In the face of unemployment and inflation, Jamaica's poorest people are not faring well.

For those who expect Caribbean islanders to be easygoing, Edward Seaga provides a bit of a shock. The Jamaican prime minister is intense, cool, controlled.

Like many Jamaicans, Mr. Seaga is also energetic and articulate. He brings to his tasks an unusual degree of discipline, working 18 hours a day when the need arises.

On overseas trips, Seaga is known to carry not just one briefcase, but three or four. The Jamaican leader has a reputation for only reluctantly delegating authority. Critics complain that he tries to do everything himself.

In addition to serving as prime minister, the Boston-born, Harvard-educated Seaga holds the important position of finance minister. At a meeting with American businessmen in the cabinet room last year, Seaga announced that he was establishing a hot line through which immediate, top-level responses would be given to foreign investors with complaints. Who would be manning the hot line? Seaga himself, of course.

Edward Seaga is not only at the end of a hot line. He's also a man in a hot seat, as any Jamaican prime minister would have to be. Seaga came to power in 1980 promising recovery from several years of economic decline. But well past the 31/2-year mark, the Jamaican leader has so far produced mixed and uncertain results.

The honeymoon is over for Seaga. People are inclined to blame him these days when things go wrong, even if it happens to be a matter beyond the prime minister's control. There's a tendency to forget that Seaga inherited a few problems, including 28 percent inflation, a lack of foreign exchange, and a cumulative drop, during the eight-year rule of the previous government, of more than 18 percent in the gross domestic product.

The heat is on Seaga for other reasons as well. He has taken a lead in the region in espousing both democracy and capitalism. With a population of 2.2 million, Jamaica is larger and more advanced in its economy than most of the Caribbean islands. It exerts a considerable influence in much of the region. A Jamaican commanded the Caribbean forces that participated in the invasion of Grenada last October. Some 40 to 50 Jamaican soldiers remain on that small island.

Almost from the outset, President Reagan pointed to Jamaica as a model for free-enterprise success in the Caribbean, a model obviously meant to be superior to the Cuban one. Prime Minister Seaga, newly elected at the time himself, was the first head of government to visit Reagan following his inauguration in January 1981.

Jamaica had just gone through an eight-year experiment with a left-leaning leadership, an experiment many Jamaicans rejected at the polls. Seaga broke Jamaica's ties with Cuba. He vowed to revive an economy that, rather than growing, was shrinking at an average rate of more than 2 percent a year.

The Reagan administration has responded by pouring more money into Jamaica than into any of the other islands. It was Prime Minister Seaga who suggested at that first meeting with President Reagan a ''Marshall Plan'' for the Caribbean and who provided much of the original inspiration for Reagan's decision to introduce his Caribbean Basin Initiative, announced in early 1982. The centerpiece of this initiative - its offer of one-way free trade for many of the region's commodities - went into effect in January of this year.

In direct economic aid from the United States, Jamaica was to receive more than $100 million a year in fiscal years 1984 and 1985. This exceeded the $91 million going in fiscal year 1985 to the Dominican Republic, which has 21/2 times as many people as Jamaica, and the $47 million going to Haiti, the poorest nation in the Western Hemisphere.

Jamaica has some advantages for businessmen, particularly for those who are in the business of tourism. Located only 75 minutes by jet to the south of Miami , it is one of the most beautiful of the Caribbean islands. According to government statistics, Jamaica's people are more than 80 percent literate. They adhere to a two-party, parliamentary system that makes sense to Americans.

But the results of increased American aid, trade, and investment in Jamaica have proven to be far from decisive so far. The one sector that is definitely looking up is the tourist trade, with a 45 percent increase in visitors over the three year period 1980-83. A Jamaican government economist said that the Caribbean Basin Initiative has stimulated dozens of new investment projects for export products as diverse as garments, light bulbs, chewing gum, street signs, and inflatable toys.

But many investment projects take time to get off the ground, and many investors are working on a relatively small scale. American investment in the island, while increasing in size, has been disappointing to many Jamaicans. The biggest disappointment for some was the failure of the US Congress to approve the tax credits that the Reagan administration proposed be granted to American firms investing in the region, said Carlton Alexander, a leading Jamaican businessman who is chairman of Jamaica National Investment Promotion Ltd., an organization designed to encourage economic recovery through private-sector investment.

Prime Minister Seaga reversed the decline in investment and economic growth. But unemployment remains high at an estimated 26 to 27 percent of the work force. Seaga sharply reduced inflation in the first two years of his administration. But in 1983 it rose to a rate of more than 16 percent and in 1984 was up to more than 20 percent - caused largely by a devaluation of the currency to spur exports. Although he has not given up, the prime minister has so far been unable to reduce the size of the bureaucracy or to cut government expenditures as much as he would have liked.

Seaga had inherited a difficult situation. Like many third-world nations, Jamaica was hard hit by the rise in oil prices in the 1970s and then by a worldwide recession. The US sharply reduced its imports of Jamaican bauxite and alumina. Seaga estimates that the drop in bauxite exports cost Jamaica $327 million in foreign exchange since 1980.

As Seaga explains it, Jamaica is shifting from a heavy reliance on bauxite to a more broadly based economy.

But the country's debt was so great that the substantial increases in loans Seaga secured from the US and from international lending organizations were to a degree neutralized by the need to pay interest on earlier loans. High interest rates have had a devastating effect. Michael Manley, the prime minister who held power from 1972 to 1980, had promised a ''third path to development,'' which was supposed to be different from both the capitalistic and the Cuban model. But this involved having a not always efficient government take control of important parts of the economy, including sugar estates. Mr. Manley's tax policies and socialist rhetoric spurred an exodus of capital as well as of skilled Jamaicans.

In the meantime, Manley drew Jamaica closer to Cuba. Most analysts say it was this as much as anything that defeated him in the '80 election. As Manley himself points out in a recent book, the fear of communism was strengthened by unofficial rationing and shortages of everything from soup to beans and chicken backs.

Manley accused Seaga's Jamaica Labor Party of working with the US Central Intelligence Agency to ''destabilize'' his government. But Seaga says that Manley and his government ''destabilized themselves.''

Seaga's great hope for reducing Jamaica's $2.8 billion debt - one of the higher per-capita debts in the world - is to increase exports. But currency devaluation and inflation were the prices the country paid for trying to make Jamaica's exports more competitive.

Under Seaga, the hand of government is still heavy. One of his projects that has the potential to increase exports - the ''Agro-21'' program - involves major governmental financing. The initial target is to use foreign advisers and modern techniques to grow fruits, vegetables, rice, and flowers and to graze cattle on 200,000 acres of unutilized or underutilized lands. The lands are eventually supposed to be sold to private owners.

The government has reduced bureaucratic controls over imports. But foreign businessmen complain that they are still beset by regulations.

Seaga's attempts to open the Jamaican economy to foreign competition have hurt small farmers, the poorest of the urban poor, and a number of businessmen, such as shoe manufacturers.

Seaga is not indifferent to those Jamaicans who have suffered more than others. In this regard, he is anything but a laissez-faire capitalist. He is introducing a massive ''food security'' program to offset the higher costs of food. It will provide lunches for 600,000 schoolchildren and food aid through health centers for 200,000 pregnant and nursing women.

Prime Minister Seaga has also devoted great time and effort to a jobs-training program called HEART (Human Employment and Research Training), which is supposed to reach more than 10,000 trainees by the end of 1984.

After describing Agro-21 as well as HEART and the food security program to a visitor, a foreign economist here concluded that ''Eddie Seaga the finance minister can't keep Eddie Seaga the prime minister from spending lots of money.''

But the very poorest of the poor have not done well so far under Seaga. A British scholar who has visited Jamaica repeatedly over the past three decades says that the slums of Kingston are spreading.

Seaga has clearly lost much of his popularity, and his reputation as a financial wizard has suffered from his encounters with Jamaica's hard economic realities. There is a feeling among some of the Jamaicans hardest hit by economic austerity that Michael Manley and his People's National Party (PNP) cared more for the poor than Seaga does. But there is also a tendency on the part of many Jamaicans to mistrust all politicians.

''I think people are beginning to realize that the politicians are not messiahs,'' says Webster Edwards, a Protestant minister who runs Operation Friendship, a skills-training program near the Trench Town slum section of West Kingston. ''I think people are beginning to realize that things are a little more complicated than that.''

''Seaga's rating has gone down,'' said Carl Stone, a Jamaican political scientist who is widely respected for the public opinion polls he conducts. ''... The mood is sort of antigovernment, but there's a sense that the PNP hasn't got all the answers either.''

That seems to leave Seaga a bit more time to run what he has described as ''a race between development and discontent.''

In the meantime, some of the things that work best here seem to be not the big projects, but the little ones nobody hears about. In ramshackle West Kingston, where nearly half the people are unemployed and lucky to get one meal a day, a US Peace Corps volunteer has built from scratch a modest training program in printing techniques for young women who are school dropouts.

At first, Sam Harle, who once ran a weekly newspaper in Mineola, Texas, had no equipment to work with. He taught English, mathematics, and bookbinding. Then when he received equipment from the US Agency for International Development, he was able to teach the rudiments of offset printing. All the women in his first class of about a dozen were able to get jobs. And Mr. Harle is trying to work himself out of a job. He is training one of his best pupils to replace him in his teaching role.

''Don't go away with the idea that it was all smooth going,'' said Webster Edwards, under whose auspices Harle has worked.

Harle was robbed half a dozen times while taking the bus or walking into the tough neighborhood where he works. But the small, wiry, cheerful former editor and World War II veteran sees some cause for hope in Jamaica. He's extending his tour in Kingston for another year. His students, incidentally, call him Uncle Sam.

Next: The Caribbean's largerst democracy, the Dominican Republic, is in deep financial trouble.

JAMAICA AT A GLANCE

Population: 2.2 million, most of them descendants of former African slaves.

Capital: Kingston, pop. 600,000.

Land: 4,240 sq. mi., slightly smaller than Connecticut. Third-largest island in Caribbean. Largely mountainous. Only 21 percent arable.

History: Earliest known inhabitants were Arawak Indians. Columbus reached Jamaica in 1494. The Spanish eradicated the Indians. In the 17th century, the English drove out the Spaniards. Jamaica became one of the world's pirate capitals and then a sugar empire worked by thousands of African slaves. Slaves revolted many times. Gained independence from Britain in 1962.

Religion: Predominantly Protestant, some Roman Catholic.

Literacy: Officially, 82 percent of Jamaicans are literate.

Major exports: Bauxite, alumina, sugar, and bananas to the US, Britain, Canada, and Norway. (Tourism replaced bauxite as the No. 1 foreign-exchange earner this year.) Marijuana is also a major export, with a retail value estimated in the hundreds of millions of dollars.

Major imports: Food, fuel, and machinery.

Labor force: 708,000 employed in 1980

Agriculture 37 percent

Industry & commerce 15 percent

Services 32 percent

Government 16 percent