The younger brother of a man who has sued a drug rehabilitation program alleging physical and mental abuse testified in Alexandria yesterday that he and his brother often smoked marijuana and hashish together and that he purchased marijuana for his brother "maybe 20, 25 times."

Defense witness George Collins, 16, also testified in U.S. District Court that he had once given his older brother, Fred, 20, a "speed pill" or amphetamine, as well as marijuana, and that his brother often bought beer for him.

Fred Collins, who has denied he had a drug habit, is suing the program for $750,000 in damages, alleging that he was held in the program against his will. A psychiatric expert testifed Tuesday that Fred Collins' use of drugs and alcohol were modest for a college student of his age and told the jury he would not have sent Collins to a rehabilitation program.

Collins, whose family lives in Fairfax County, claims he was "imprisoned" for four months in the Straight program in St. Petersburg, Fla.

Straight's attorneys say Collins admitted himself to the program voluntarily, and deny any physical or psychological mistreatment. Testimony in the case was completed yesterday and the jury is expected to begin its deliberations today.

Another defense witness, Miller Newton, national clinical director of Straight, told the jury, "I would not want to hold Mr. Collins or any other adult in the program against his will." In response to testimony presented Tuesday that Newton grabbed an Oxon Hill woman in the program by the hair and threw her to the floor, Newton said that the woman "had her nails in a couple of clients and her teeth in another" when he intervened.

"I took her by the hair and the waist . . . which is a recognized technique," said Newton.

"Where did you learn these techniques?" Collins' attorney Philip J. Hirschkop asked him. "Were you a commando or something?"

"I resent that remark," Newton responded.

Newton also said that "marathoning," intensive rap sessions that sometimes lasted 70 hours, went on during Collins' stay in St. Petersburg, but said the practice had been discontinued.

Dr. Robert L. Dupont Jr., clinical professor of psychiatry at Georgetown University medical school, testified that based on his reading of court documents, he believed Fred Collins was drug dependent when he was admitted to Straight last June. Dupont said Collins' continued use of beer and marijuana after his father's threat to cut off financial support showed he was a "sick, pathological user."

Dupont also acknowledged that he has been a consultant to Straight for four years and that Straight has paid him $7,000 so far for his work on defending the group against Collins' lawsuit.