More Americans are living longer than ever before, but the rapid growth in the elderly population is putting a "great stress" on the health care system, the nation's top health official said yesterday.

In releasing an annual update on health in the United States, Dr. Edward N. Brandt Jr., assistant secretary for health in the Department of Health and Human Services, concluded that there is "ample evidence that the level of health in this country is good and getting better."

The 337-page document does indeed show that infant mortality continues to drop, life expectancy at birth is increasing and death rates are declining for nearly all major causes of death, including heart disease and stroke.

But, noted Brandt, the figures are not all favorable. Black infants still die at nearly twice the rate of whites in the first year of life, minorities in general can expect shorter life spans, and lung cancer will soon replace breast cancer as the leading cause of cancer deaths among women.

There were other reversals. While there had been a "substantial decrease" in car-accident deaths in the early 1970s, the rate increased about 10 percent in a three-year period later in the decade. The jump, which is particularly high for white men in their teens and early 20s, may in part be due, said one health analyst, to the national shift toward smaller cars.

Expenditures for health were also high, reaching $247 billion or an average of $1,067 per person in 1980, up 15 percent over the previous year's and nearly doubling between 1975 and 1980.

The analysis of health status, resources and expenditures compiled by the National Center for Health Care Statistics and other agencies included the following:

* Life expectancy at birth exceeds 73 years. Women still have the edge, living more than 77 years compared to 69.5 years for men in 1978. Although the life expectancy charts show a "blip" downward for 1980, health experts attribute this to the year's flu epidemic and say the longterm trend still seems to be up.

* Heart disease and stroke deaths have continued to decline for all ages. The cancer mortality rate has dropped for those under 50 years of age because they are smoking less and because the forms of cancer the young get tend to respond better to treatment. It has increased among older people, primarily in lung cancer, which probably reflects the fact that many older people have smoked for years.

* The population 65 years and up rose from 20 million to 25 million during the 1970s and is projected to reach about 35 million by the year 2000. The elderly obviously are the greatest users of medical care, and nursing home expenditures doubled between 1975 and 1980.

* Americans are more conscious of good health practices. The numbers of women examined for breast and cervical cancer have increased and more people are getting blood pressure checks. However, those with less education are less likely to have good health habits, particularly in regard to smoking, drinking alcohol and using seat belts.

* Between 1974 and 1979, tonsillectomies for children dropped by 40 percent, but the rate of Caesarean births jumped 78 percent.

* Personal health care expenditures more than tripled between 1970 and 1980, with two-thirds of the tab picked up by public and private health insurance plans.

"Overall, we have had a good return on this investment in health. What we are seeking now are ways to improve the return, to constrain the increases in health expenditures without jeopardizing our level of health," said Brandt.

An outside critic, however, raised the question of the future in health.

"The credit for any good news about the present state of health in this country must go to past commitments to prevent death and illness and to provide health care to many who previously got none," said Dr. Sidney Wolfe, director of the Nader-backed Health Research Group.

He contended that "at the very time they are announcing this good news, the Reagan administration is unleashing a series of plagues upon this country"--changing regulations and reducing government programs--which will lead to an "inevitable" worsening in overall health.