Results of Brazil's decennial census show population growth rates have dropped to the lowest level in 40 years, dimming nationalist dreams of surpassing the United States as the Western Hempisphere's most populous nation by the end of the century.

The recently released head count surprised population experts who believed the Latin American giant had more people than the 119 million tallied.

According to Thomas W. Merrick, director of Georgetown University's Center for Population Research and an expert on Brazilian demographics, Brazil's population grew at an annual rate of 2.4 percent, significantly less than the 2.7 percent that had been predicted.

"I had once guessed that Brazil would catch up with the U.S. in population sometime in the early 21st century," Merrick said. "And I suspect that would still be the case." The 1980 census recorded the U.S. population at 226 million, about 3 million more than expected.

Although two-thirds of the Brazilian population is concentrated along the eastern seacoast, pioneers pushing into Brazil's Wild West -- the Amazonian wilderness -- have made the region the fastest growing in the country. During the 1970s, the northern territory of Roraima doubled in population to 79,000, and the boom-town, western territory of Rondonia quadrupled to 500,000.

Attacted by gold strikes or the simple lure of a homestead, the immigrants on the Amazonian frontier bucked a nationwide trend -- the census found that the Amazon was the only region in the country where men outnumbered women.

Designed as a gateway to the west, Brasilia is the nation's fastest growing city, doubling in a decade. Laid out on the high plains of the interior, the country's futuristic capital last year reached the 1 million mark -- the level once forecast for the year 2000.

In contrast, a string of southern towns lost inhabitants, and, as in the United States, angry mayors have demanded recounts for fear of losing state and federal funds.

The hardest hit was Parana State in southern Brazil, where in 1975 a "black frost" killed coffee trees, causing a quintupling of coffee prices and leading to boycotts in American supermarkets. Registering the smallest growth rate of Brazil's 27 states and territories, Parana lost 90,000 residents, mostly to the highly developed Rio-Sao Paulo corridor.

"By the year 2000, I can foresee a megapolis of 45 million along the Rio-Sao Paulo axis," Jesse Montello, census director and president of the Brazilian Institute of Geography and Statistics said in an interview.

Roughly the same length as the Washington-New York corridor, the Brazilian coastal corridor has at one end the Rio de Janeiro metropolitan area with 9 million people and at the other end the Sao Paulo metropolitan area with 12.5 million inhabitants. With that population, the Sao Paulo region would be among the world's five largest.

Montello and other demographers attribute Brazil's declining growth rate to increasing literacy, more working women and an increased exposure of the general population to "modern" life styles -- either through emigration from rural areas to cities or through television, which reaches into virtually every corner of Brazil.

Although the annual growth rate has fallen from a 1960 high of 3 percent to 2.4 percent today, one-half of Brazil's population is under 18, and demographers fear a continued, steady expansion. Each year 3 million people -- or almost the entire population of Israel -- is added to Brazil's population.

Brazil is the only major Third World country without a nationwide family planning progam. However, in light of mounting economic problems, the ruling military leaders are moving toward seeking to limit births.

In December, Health Minister Waldry Arcoverde unveiled a program of free contraceptives, free sterilization and free tubal ligation to be administered through existing state-run medical clinics.

Three years ago, the government was forced to shelve a similar program because of a political fight with the powerful Roman Catholic Church over a divorce law. This time, the birth control proposals have been repackaged as "family planning," and strong populist touches have been added.

"We want to end the monopoly of information where the most favored classes can plan their family to the size they wish, while the most needy remain in ignorance," Arcoverde said.

The church reaction was not long in coming.

"An affront to the Christian conscience," said Cardinal Eugenio Salles of Rio.

"Incredible and intolerable," protested Archbishop Don Helder Camara of Recife.

The church has received backing from many nationalist-oriented medical associations, which charge that multinational corporations make and distribute birth control pills in an effort to undermine the country's future.

"To be a superpower, a country needs a large land mass, abundant resources and a large population," Mario Victor Assis de Pacheco, a gynecologist and secretary general of the 1,500-member Rio medical association said in an interview. "Brazil has everything but the population -- we need a minimum of 300 million people."

However, government planners, who privately scorn such scenarios as the "Indianization" of Brazil, are set on a family planning progam. Arcoverde replaced a health minister viewed as too timid on the issue. Last year an ultranationalist general spoke out sharply against family planning. He was stripped of his command.

Despite the church, the doctors and the government, Brazilian couples are already deciding how many children to have. A Gallup poll released last month said that 71 percent of those tend to limit their families. The number of children regarded as ideal for a family has dropped from three in 1977 to 2.5 today.

In 1978, a separate study of married women in Sao Paulo found that two-thirds were using contraceptives. Of those who did, only 1.3 percent cited religious reasons for not doing so.