Fueled by high birth rates among Latinos, this most populous U.S. county has entered a new period of explosive growth that has surprised demographers and pushed Anglos into minority status for the first time in this century.

The surge, which studies show has sent Los Angeles County's population above 8 million, has also caused policy makers concern about finding space and funds for a booming Latino community, which includes many illegal immigrants.

In the early 1970s, the postwar population boom that made the county a national symbol of suburban sprawl, seemed to have exhausted itself, leading several experts to predict much slower growth in this decade.

Instead, studies show that the county is adding nearly 120,000 residents each year. "I kept expecting to see it taper off, and it did not," county population studies chief George Marr said.

For years, Los Angeles has been heralded as a magnet for job-hungry immigrants from Latin America and Asia, but experts said the influx of newcomers is only indirectly responsible for the population spurt.

Most of the growth, according to Marr and state government population analyst Elizabeth Hoag, has come from a baby boom among young Latinos and, to a much lesser extent, ethnic Asians and blacks.

That has led the Los Angeles Area Chamber of Commerce to estimate that non-Latino whites comprise only 46 percent of the county population, down from 53 percent in the 1980 Census. Marr said that such estimates are tricky, but that he is confident that the Anglo population is no higher than 50 percent and certain to have minority status soon.

According to Chamber of Commerce estimates, Latinos now comprise 31 percent of the population, up about 3.5 percent from 1980.

Ethnic Asians and Pacific Islanders have climbed from about 6 percent in 1980 to 10 percent currently, while blacks have remained at about 12 percent. Demographers have predicted that Anglos will be a minority in the state within 50 years.

The growth that has made Los Angeles "a true polyglot center" shows no sign of abating in this decade, Marr said, leaving city and county officials to wonder how they can handle sizable numbers of new hospital patients and school-children.

Los Angeles County Supervisor Michael Antonovich noted that one recent survey showed 73 percent of babies born at the publicly funded USC Medical Center were members of illegal immigrant families. "We have very serious problems with illegal aliens," Antonovich said.

But he hailed the new population figures as a sign of the county's continuing economic prosperity and predicted that passage of a stalled immigration-restriction bill in Congress would ease the problem of illegals.

Most of the new growth can be explained by the unusually large percentage of Latino women of prime child-bearing age and by the fact that large families traditionally are preferred by Los Angeles residents of Latin-American heritage, according to Hoag. "You have a lot of young women having children right now," she said.

Fertility rates among Asians and blacks are also high, although the birth rate among blacks, like that of whites, appears to be declining. The number of Anglo births each year here roughly equals the number of Anglo deaths.

State figures show that the post-1980 population boom has affected other areas that languished in the 1970s. San Francisco lost 37,000 residents during the last decade but has gained 56,000 since 1980 as immigrants have arrived from Asia and Latin America.

One former boom area, Orange County, has experienced such slow growth that it has been overtaken by San Diego, with a population of 2.13 million, as the state's second most populous county.

Population experts said Los Angeles' growth is particularly startling given the large number of residents leaving the county each year, many apparently seeking less crowded, less expensive neighborhoods in outlying areas.

According to figures compiled from driver's-license changes, nearly 198,000 people left the county last year and only 175,000 moved in. But these figures do not include thousands of unlicensed young immigrants arriving annually.

Demographers said they are reluctant to project population trends here beyond a few years, but Hoag cited factors that might slow growth.

Birth rates are expected to drop, she said, and the temporary surge of Indochinese refugees is declining. "Of course, if the Philippines flared up into a civil war, it might start up again," she said.