While American science enjoys excellent health, the National Academy of Sciences said in a massive report to Congress this week, it no longer dominates the world the way it did 20 years ago.

"The remarkable preeminence of the United States in science cannot sustain itself," academy President Philip Handler said in sending the 544-page report to Congress. "We are still a productive nation, but we're seeing the emergence of other countries in ways that allow them to do things that only we were able to do before."

In 1955, Handler said, Americans were responsble for three-fourths of the scientific endeavor around the world. The percentage has dropped dramatically. Today, Handler went on, the United States accounts for no more than one-third of the world's science.

"That one-third is still the most productive one-third," Handler said, "but countries like the Soviet Union, Japan, Germany and France do quite well in research and are likely to continue to do quite well in research."

Despite losinig its dominant role in world science, the academy report to Congress said, the United States still ranks first in many fields, including biology, space science, astronomy and computers. The advances in these four fields have been explosive in the last decade, the academy said, and it predicted that breakthroughs will continue.

The most dramatic strides in medicine have been made in genetics, the academy said, where Americans have won so many recent Nobel Prizes. Breaking the genetic code is only the beginning of what promises to be what the academy called an "explosive" science for years to come.

Noting that specific defects have been identified in more than 150 hereditary diseases like cystic fibrosis and sickle-cell anemia, the academy study said scientists are on the threshol of understanding other genetic ailments like rheumatoid arthritis by learning which genetic markers determine the disease.

"Lymphocyte [white blood cell] and tissue typing of humans has barely begun," the academy said. "It is clear that as an increasing variety of tissue antigens is typed in larger numbers of individuals, additional correlations of this character will surely be found. They perhaps will predict which of us are most likely to develop cancer, atherosclerosis, multiple sclerosis and so on."

Even more predictable, the report went on, are the contributions genetics will make to the harnessing of biological processes for human welfare. The academy pointed out that recombinant DNA methods are already being used to convert bacteria into factories that make insulin and a powerful anti-viral agent called interferon that can help ward off disease.

"Another consequence [of genetics] might be gene therapy -- the correction of genetic diseases by replacing defective genes with normal ones," the report said. "It seems likely that this step may become possible before too many years for the defective precursors of blood cells, since these are located in the bone marrow in a way that allows them to be replaced from the bloodstream."

The academy report said that astronomy is just as explosive a science nowadays in the United States as biology.

"The story of astronomy is the most exciting story in physical sciences," said the Naval Research Laboratory's Dr. Herbert Friedman, who chaired the academy study on the state of astrophysics in the United States. "It's rare that a month goes by without some strange new object being discovered that challenges everything we know about physics."

Friedman said the most recent discoveries have been of objects in the heavens that suddenly burst forth with gamma rays with a force of 1 million suns for a few seconds, then disappear from sight forever. At the same time, astronomers have found clusters of 1 million stars that flare the x-ray output of 1 million suns into the heavens, turn themselves off after a few seconds and then begin to burst x-rays again a month later.

"There is one x-ray buster that turns itself on and off thousands of times a day," Friedman said, "and others that repeat every few days, weeks and months. It's all quite remarkable."

Friedman said it's impossible to predict new astrophysical discoveries, except to say that they will continue to stretch our imaginations. He said: "one thing you can say about astronomy. We learn a great deal and understand very little."

Computer science is still one of the most explosive techologies in America, the science academy report said, even after two decades of remarkable growth. The academy predicted that the microcomputers of the type that control cooking times and temperatures in microwave ovens and ajust the flow of fuel to automobile engines will mushroom so fast in the next few years that there will be "millions" of them in use throughout the United States in the next decade.

"The microcomputer of the mid-1980s will be capable of truly impressive performance," the academy said. "It will not only extend the range of economic computer applications, but will become a major workhorse of the future."

A few trends the academy spotted in American science were worrisome, the report said. One is the cost of doing research in the United States, where it now takes more than $100,000 for the complex laboratory instruments for every new investigator. Another is the decline in the number of trained doctors going into medical research.

"The percentage of MDs among those applying to the National Institutes of Health for research support declined from 41 percent in 1966 to 28 percent in 1976," the academy said. "If this trend continues, it could undermine the transfer of basic research findings to clinical practice, while research itself might be denied the special insights arising from human pathology."