A study released this week by four Harvard doctors concludes that heavy marijuana use does not invariably cause shrinkage of the brain as British researchers have previously asserted.

The study challenges the results of a team of British researchers who claimed in 1971 that regular marijuana use causes structural changes in the brain, Dr. John C. Kuehnle, assistant professor of Psychiatry, said yesterday.

Kuehnle, who is also clinical director of the alcohol and drug abuse research center at McLean Hospital in Belmont, said the 1971 study is not totally valid since the painful process used in the testing, pneumoencephalograms, involves injecting air into the brain and could conceivably cause further brain damage.

Because they used this process the researchers were ethically obligated to use subjects with previous neurological disorders, which further invalidates their results, Kuehnle said.

The Harvard study involves 19 individuals who were already involved in a marijuana study at McLean Hospital, Kuehnle said. Since the subjects were previously under supervision, doctors could be sure they smoked between 50-160 joints in a 21-day smoking period.

Dr. Paul F.J. New, associate professor of Radiology, performed the radiological aspects of the study at Massachusetts General Hospital.

The implementation of a computerized X-ray technique, the CAT scan, constitutes a major breakthrough in research methods, New said yesterday.

This safe and painless method delineates the brain by feeding X-ray results into a computer, New added.

Although the testing showed no structural changes in the brain, the doctors admit there is a possibility of extremely minor structural changes that cannot be detected by current medical methods, New said.

This study will probably have little influence on the question of marijuana legalization, Kuehnle said.

"This is one piece of a large mosaic of data and not a general approval of marijuana use by any means," he said.

New said the major significance of the study is that it proves the changes seen by British doctors six years ago could not have taken place.

New added, however, that the number of participants in the study was small and "cannot be considered conclusive evidence for the safety of marijuana use."

The other two Harvard doctors involved in the study are Dr. Kenneth R. Davis, assistant professor of radiology, and Dr. Joel H. Mendelson, professor of Psychiatry.