The debate on legalizing narcotic drugs grinds on

Cameras flashed and world headlines blazed when pop singer Paul McCartney and his wife Linda were fined recently for bringing marijuana into London. Defiantly they told crowds of waiting newsmen that possession should be legal.

Soon afterward, American movie actor Anthony Perkins, star of 40 films, was fined for bringing into the United Kingdom a small quantity of marijuana and three doses of the powerful hallucinogen LSD. He claimed he had hurt no one. He felt he had been taken to court only because he was in the public eye.

Yet the young Marquis of Blandford, Jamie Spencer-Churchill, heir to Blenheim Palace, recently warned young people to stay away from heroin. He continues a two-year struggle to break his own addiction.

Cases like these have reopened one of the most contentious debates in modern society:

Should narcotic and stimulant drugs such as marijuana, LSD, heroin, and cocaine be made legal? Or should they continue to be banned as menaces to health and society?

Entertainment figures such as former Beatle McCartney and Hollywood star Perkins demand the freedom to take marijuana and other drugs privately.

Many in the US and Europe support legalization on two more grounds:

* To try to undercut massive illegal traffic in drugs worldwide.

* To try to strip hard drugs and marijuana of their glamorous image as prohibited substances in order to, hopefully, reduce their appeal to young people.

But the majority of experts contacted by this newspaper in recent weeks reject these arguments.

They insist that legalizing would simply send a message to young people that ''drugs are OK.'' It would open up opportunities for more people to try so-called ''starter drugs,'' such as marijuana, that often lead on to harder varieties.

Besides, the experts say, legalizing would not wipe out the black market but would simply intensify it for harder and more exotic drugs. Traffickers would use any country that did legalize as a base of smuggling operations against those that did not.

To many officials, parents, and others, legalization remains ethically and morally wrong.

Most countries follow the US practice in outlawing heroin for all reasons, and of strictly controlling other dangerous drugs under doctors' prescriptions.

The UK is the only major country where physicians may prescribe heroin for addicts at treatment centers. Many Britons shy away from heroin nonetheless, believing the range of alternative analgesics is adequate.

In arguing the case for legalization, Paul McCartney says: ''Cannabis is a whole lot less harmful than rum punch, whiskey, nicotine, and glue, all of which are perfectly legal (in the UK).''

Mervyn Manby, retired former senior official of the United Nations Division of Narcotics Drugs, says: ''International policy is American policy writ large.

''Any consumption of drugs apart from medical or scientific purposes is held to be wrong. . . . Perhaps it's time for a change. The drug situation is getting worse. . . .

''Perhaps we ought to allow tobacco companies to sell mild marijuana cigarettes, highly taxed. It might take away the illicit market.'' He agreed that many might still turn to the black market for stronger varieties.

''As for cocaine, couldn't we produce a weaker strain? Couldn't it go back into Coca-Cola, where it was before the Harrison Act in 1914 outlawed it?''

According to Kevin Zeese of the National Organization for the Reform of Marijuana Laws (NORML) in Washington, each individual has the right to decide what enters his own body.

''Besides,'' he said in an interview, ''(American) high schoolers can find marijuana freely today. Illegality just glamorizes it. Legalization would allow controls on the quality of marijuana sold.'' Taxes, he said, would raise at least $10 billion a year. Democratic sources in Congress put the maximum figure at $3 billion.

Mr. Zeese accepted that marijuana ''affected'' the lungs. NORML, he said, would support a government health warning on marijuana packets, but he denied medical evidence that marijuana muddies consciousness and slows physical growth.

Meanwhile, experts including Prof. Arnold Trebach of American University in Washington, D.C., want marijuana and heroin legalized under strict controls as medical aids to patients who cannot receive relief in any other way.

The case against legalizing:

''Legalization is a canard,'' says Mrs. Tamar Oppenheimer, director of the UN's Division of Narcotic Drugs in Vienna.

''You don't legalize crime or theft simply because their numbers are rising everywhere. It's illogical. . . . As for medical uses, well they're a drop in the bucket compared to illicit use. . . .

''Drug trafficking and abuse are not 'victimless' crimes. They hurt the user, his family, and all of society. . . . The question is, where do you balance individual rights and the public good?'

Michael Davies, one of her senior deputies in Vienna, adds, ''If we were considering today whether to legalize alcohol and tobacco, we would be against them, because we now know the damage that both cause to health. . . .''

A senior medical officer at the British Department of Health and Social Security in London agreed: ''Why should we legalize marijuana, when we already have enough harmful substances to cope with?''

Another problem is seen by the chief Canadian police officer in charge of fighting drugs, Superintendent Rod Stamler:

''We license alcohol,'' he said. ''But we regulate it as well. We have Breathalyzer tests for drivers. But we have no such quick, effective tests for drivers who have taken marijuana and cocaine.

''We know that marijuana distorts both reflexes and the judgment of time and distance.''