MINNEAPOLIS, Aug. 7— A nationally known child psychiatrist has been convicted by a Federal jury of falsifying data in a $250,000 drug study at the University of Minnesota.

Dr. Barry Garfinkel, who is widely respected for his work on suicide among teen-agers, was found guilty on Thursday after a two-week trial in which a former assistant testified that he had told her to "make up" data. The verdict makes him one of the few American scientists ever convicted of a crime in connection with a study of an experimental drug regulated by the Food and Drug Administration.

Dr. Garfinkel, 46, was accused of falsifying reports during a study of Anafranil, an anti-depressant that he tested from 1986 to 1989 on patients with obsessive-compulsive disorders. His research was part of a national study to prove that Anafranil was safe and effective enough to be sold in the United States Drug Approved

The drug was later approved by the Food and Drug Administration, but without most of Dr. Garfinkel's data, which were scrapped after the accusations came to light.

Dr. Garfinkel maintained that he had never set out to defraud anyone and had merely made honest mistakes while running the study.

But the prosecutor, Assistant United States Attorney Andrew Luger, said that Dr. Garfinkel had treated the study "like a joke," faking reports on patient examinations that had either never taken place or had been conducted by someone with no medical training. In the study, patients were supposed to come in for regular medical visits to determine if the drug was helping or harming them.

Mr. Luger, a former Brooklyn prosecutor, portrayed Dr. Garfinkel as an ambitious scientist who had enrolled more patients than he could handle, and had then lost interest in the detail work of the study, leading to a "three-year pattern of fraud and deceit." In one instance, according to testimony, Dr. Garfinkel had reported conducting an examination on a girl when he had merely been riding in a car with her mother.

Dr. Garfinkel admitted he had not examined all the patients himself, but he said he had been able to obtain the necessary information in other ways.

His lawyer, Doug Kelley, argued that his client was a "caring and compassionate man" who had "no motive whatsoever" to commit a crime, because he never got a penny from the study himself.

The drug's manufacturer, the Ciba-Geigy Corporation, paid the university $250,000 to conduct the research. Dr. Garfinkel was the director of child and adolescent psychiatry at the university until he was indicted in February. He is still an associate professor in the medical school. Testimony From Assistant

Dr. Garfinkel blamed his study coordinator, Michelle Rennie, for filling out the false reports. Ms. Rennie, in turn, testified that Dr. Garfinkel had ordered her to make up data on patient eye examinations and physicals that were never conducted. She reported the practice to the university in 1989.

Last Thursday, Dr. Garfinkel sat in silence as the jury in Federal District Court here pronounced him guilty of two counts of mail fraud and three counts of filing false statements. He faces up to five years in prison for each count when he is sentenced. He was acquitted of 18 other charges. Judge David Doty, who heard the case, will sentence Dr. Garfinkel in about two months.

Prosecutors admit it is rare to seek criminal charges against a scientist in such a case. Since 1978, only 24 doctors, including Dr. Garfinkel, have been convicted of crimes involving tests of new drugs, said Mark Brown, the associate chief counsel for the Food and Drug Administration, which oversees all such research. In most cases of improper conduct, scientists merely face restrictions on doing further drug tests.

"The decision to go forward with a criminal charge really concerned Dr. Garfinkel's arrogance in faking patient visits," Mr. Luger said. "Falsifying the very existence of patient visits amounts to fraud."

Defense lawyers said that the evidence did not support the verdict and that they would ask for a new trial.

Both sides said the verdict would be watched closely by other scientists.

"The case was not brought to send a message to anyone," Mr. Luger said. "But it is our hope that the verdict here will send a signal loud and clear that wrongdoing during these important drug studies will not be tolerated."

Mr. Kelley, the defense lawyer, said the verdict was likely to send a far different message. "I think my problem with this case from Day 1 has been that they turned a breach of the protocol into a crime," he said. "And scientists breach the protocol in many many studies all the time. If this case sets a precedent, there will be a lot of prosecutions."