Mexico City — Sofia Salinas Ugalde has lived in Netzahuacoyotl, the capital's worst slum, for almost 10 years. Soon, however, she will return to her hometown in the southern state of Oaxaca because she ''can't stand'' life in the mostly unpaved, rat-infested labyrinth of concrete buildings where she now lives.
Mrs. Salinas Ugalde and her husband have put their modest home and family business (a meat stand) up for sale and hope to leave as soon as possible.
Indeed, the Mexican government would like more people to abandon the capital. For two months now, posters showing idyllic watercolors of a small town have adorned the city's subway system. Their message is that passengers should think of taking another trip - a permanent journey back to the provinces.
''There, you'll be able to share your breakfast with your family, leave together for work or school,'' say the posters. ''In the provinces, families live together.''
The posters are one part of a new national urban development plan launched by Mexican authorities. It is aimed at keeping peasants on their farms and redistributing population growth to medium-sized cities to check Mexico's unruly urbanization.
The plan's first phase, which will last until 1988, was made public recently. It came in response to increasingly gloomy projections by national and international experts that, despite government efforts, Mexico's urban growth is out of control.
Last July, the World Bank released a report predicting that by the year 2000, Mexico City would become the world's largest metropolis, overtaking Shanghai, with a population of 31 million. Mexico's population as a whole would jump from 84 to some 100 million, the report said.
With an estimated 17 million inhabitants, the Mexico City metropolitan area is already one of the largest in the world. Heavy smog from industries concentrated in the capital, snarled traffic, haphazard concrete buildings, and excessive noise levels are present realities and presage further problems, should development continue unchecked.
According to the 1980 census, Mexico's urban population accounts for two-thirds of its inhabitants. That proportion is expected to reach 75 percent by the year 2000.
''Twenty-six percent of Mexico's population lives in the metropolitan areas of Mexico City, Guadalajara and Monterrey,'' President Miguel de la Madrid Hurtado said recently. ''In contrast, the rural population consists of 22 million people who are scattered in 123,000 villages of fewer than 2,500 inhabitants each.''
The new plan ''is intended to foster the control and decongestion of large cities and the regrouping of scattered settlements,'' he added. ''Thus the promotion of medium-sized cities plays a major role in the program.''
According to Urban Development and Ecology Secretary Marcelo Javelly Girard, ''Fifty-nine cities have been selected with the help of state officials ... to absorb the demographic and economic growth of the nation.''
The new plan will also create national parks and improve the nation's water supplies, infrastructure, and public services, the secretary added.
The government's heavy spending in highly populated areas in the past has often been blamed for encouraging people to move to big cities. But under the new plan, more funds will go to smaller towns.
''For the first time in Mexican history, disbursement and public investments by federal and state authorities are being used in population planning,'' said Geronimo Martinez, general secretary of the National Population Council.
''The government will give cities the infrastructure they need so they will be attractive to migrants. This in turn will help cities keep people by giving them the services they expect in education, health, housing, and employment.''
Two factors will probably help the new plan. First, large cities have become less attractive to migrants because of Mexico's economic crisis. Unemployment is high and the migrants fear they will not be able to find work.
Second, as living conditions worsen, Mexico City residents are more willing to leave the capital.
''I wouldn't go back to Mexico City for anything in the world,'' said Raul Herrera Moreno, food and beverage manager at the Hotel Club Lagoon Caribe in Cancun, a resort on the Caribbean.
''I used to live there before I came to Cancun. I went back for three days recently and I could hardly breathe because of the pollution,'' he added.