When 350 pregnant mice were exposed to modest but repeated doses of microwave radiation recently in a government laboratory, seven of their 3.500 fetuses were found to have a severe malformation that left their brains protruding from their skull tops.

The abnormality, known as exencephaly, was statistically significant, scientists say, because no such malformation occurred in control group of mice of exposed to radiation.

Experimental results such as this are stirring increasing concern among scientists and government officials over whether microwaves and other forms of radiowaves may pose a health hazard at relatively low intensities. They also wonder if additional safeguards may be needed to protect the American public.

The issue, many researchers say, raises troubling and unanswered questions. The biological effects of microwave and other radio-frequency radiation remain poorly understood. And some government officials say scientists are "years away" from untangling the enigmas.

With the continuing growth of microwave communication links such as telephone and television relay systems, radio-frequency radiation is becoming increasingly pervasive, creating what some scientists term a new form of environmental pollution.

Only a small percentage of the public is apparently exposed to more than a minimal level of radio-frequency radiation, but researchers have yet to establish how much exposure this small fraction of the U.S. population - possibly several million - undergoes.

While government officials are seeking to determine whether low-intensity radio-frequency radiation poses any hazard, some scientists also say they hope to discover whether such radiation has additional beneficial uses, possibly in medical therapy.

The lab finding of abnormalities in mouse fetuses was one of several recent research results that prompted a key government research official to tell a Senate committee last month that American scientists are beginning to turn up evidence that appears "confirmatory" of earlier Soviet studies of biological effects - some of them apparently adverse - caused by microwaves.

Soviet and Eastern European nations - which have set far more stringent safety standards for microwave radiation than the standard adopted by the United States - have pioneered in studying biological effects of low intensity radio-frequency radiation. American scientists have begun focusing research on low microwave radiation levels only in the past few years.

"We've going to have to lend more credence to th voluminous literature that has been developed in those countries." Daniel Cahil, experimental biology laboratory chief for the Environmental Protection Agency, told the Senate Commerce Committee during recent hearings on radiation.

While the experiment on mouse fetuses, performed at an EPA research lab in North Carolina, appeared to bear out Soviet reports that microwaves may have biological effects at moderately low intensities, the finding, like many others so far, raised many uncertainties.

The results remain to be further verified through laboratory efforts to duplicate them. If a link between microwave radiation and abnormalities in mouse fetuses is confirmed, researchers would still not know why the radiation led to the malformations.

Ezra Berman, the scientist who conducted the experiment, said moreover, that no inference could be drawn from his lab findings about how similar radiation might effect human fetuses.

Microwave radiation has been a focus of recurrent controversies, ranging from debate over the safety of microwave ovens to international exchanges over Soviet microwave bombardment of the American Embassy in Moscow.

For many researches, microwave radiation is a far more significant and pervasive phenomenon because of its widespread use in television and telephone relay systems, military communication links, radar medical technology and numerous industrial devices.

Researches, moreover, are concerned not only about microwaves but also about other radio-frequency waves, now permeating the atmosphere because of radio and television transmission.

Microwaves and other radiowaves form a part of the electromagnetic spectrum. Their characteristics differ markedly from higher-frequency radiation such as x-rays. Microwaves and other radio-frequency radiation are not emitited by radioactive substances and do not cause radioactivity in substances exposed to them.

Microwave radiation has long been believed to cause injuries such as cataracts and internal burns at exposures of high, or acute, intensities. What researchers are now seeking to determine is whether harmful effects may also occur at low levels.

Evidence is gradually accumulating though it is fragmentary and its implications are nuclear.

In another experiment at the EPA lab in North Carolina, Ralph Smialowiez, a research microbiologist, has turned up preliminary evidence of changes in the blood of rats exposed repeatedly to moderately low levels of radio-frequency radiation. The rat's blood composition changed slightly and blood cell responses appeared altered, apparently confirming similar Eastern European findings.

While the results were satistically significant, Smialowiez said, their implications cannot yet be assessed.

John H. Heller, a research scientist for the New England Institute in Ridgefield. Conn., has reported finding abnormalities in chromosomes of hamsters and fruit flies exposed to low levels of radiowave radiation. Chromosomes are carries of genetic information, which determine heredity.

The chromosomal abnormalities. Heller said, "seem to be out of all proportion" to the law level of radiation used in the experiments.

"I, for one, do not understand why in our own experiments we got the types of results that we saw," Heller added.

Allan H. Frey, a biophysicist for Randomline. Inc., a research firm in Huntingdon Valley, Pa., had reported a series of experimental findings in animals exposed to low levels of radio-frequency radiation, including effects on their nervous systems.

In one experiments, Frey said, chemical substances were found to penetrate the brains of rats after the rats were exposed to radiation. In the absence of radiation, the same substances would not penetrate rats' brains. Another finding was changes in frogs' heart beat rates.

"But the observation of effects does not necessarily mean the finding of a hazard," Frey said in a research paper. "Thus, due to the lack of data. We are left with a large question mark."

Russell L. Carpenter, a research biologist for the Food and Drug Administration's Bureau of Radiological Health, has demonstrated in lab experiments that cataracts will form in rabbits' eyes, but only when exposed to relatively high levels of microwave radiation.

Carpenter's findings, nonetheless, raise significant questions. His experiments indicated that microwave radiation may have a cumulative effect - an effect caused by repeated, short doses of radiation rather than a single long dose.

His findings also indicate that the overall heating effect of microwaves would not in itself explain why cataracts had developed. "So it makes a nice puzzle," Carpenter said.

Few studies have been conducted by American researchers about whether low-level microwave radiation may affect humans.

Milton M. Zaret, a Scarsdale, N.Y., opthalmologist who is among the most controversial figures in the microwave research field, said in an interview that he has examined more than 100 patients suffering from cataracts and several hundred patients with other eye injuries, all resulting from exposure to microwave radiation.

Several of these injuries, he said, were caused by exposure at low radiation levels - intensities considerably below the U.S. occupational safety standard.

"My assumption is that the (U.S. safety) standard is wrong," Zaret said. Zaret has also asserted there is a possibility of a relationship between microwave radiation and cancer. Other American scientists have expressed skepticism about Zaret's clinical findings, arguing that adequate evidence is not available to support his conclusions.

As a result of the controversy over microwave radiation at the American Embassy in Moscow, a study of all personnel who were stationed at the embassy since 1953 is now being carried out by Abraham M. Lilienfeld, a Johns Hopkins University professor of epidemiology.

Lilienfeld said he does not expect to have results before spring.

Soviet and Eastern European researchers have reported a variety of effects on nervous systems of people as well as laboratory animals exposed to radio-frequency radiation.

Their findings include changes insensory responses and in brain wave activity, as measured by electroencephalograms - recordings of the brain's electrical activity.

Other Soviet and Eastern European findings have ranged from reduced heart beat rates to headaches, sleep disturbance, memory changes and depression. American researchers have expressed uncertainty about validity of some of the Soviet and Eastern European reports, though these findings now appear to be gaining increasing acceptance here.

Frey, who has recently returned from a 3 1/2 week visit to the Soviet Union where he talked with Soviet scientists, said in an interview that the Soviets continued to conduct a "far larger" research program to probe the biological effects of radiowaves than American scientists.

He praised the Soviets as knowledgeable, perceptive and competent researchers and said they are focusing on some of the issues that puzzle American scientists.

Among the enigmas are explanations for the mechanisms through which low-intensity microwaves may cause biological effects. It is unclear whether such effects occur because of some form of localized heating or conceivably, through electrical or molecular changes.

Scientists also say they do not understand the way in which frequencies and wave lengths of radio-frequency radiation may be linked with the biological effects the radiation apparently causes.

Does what is known about the effects of microwaves and other radiowaves suggest that people may be endangered even at the extremely low levels that permeate a country like the U.S.?

"Is it possible and conceivable? Sure. But tat doesn't mean it's likely," Heller, the researcher who found chromosomal abnormalities in radiowave experiments, replied in an interview. "I certainly would like to see the experiments done. They need to be done - andneed to be done badly."