David Berkowitz, charged in the "Son of Sam" killings, today began undergoing a battery of psychiatric tests to determine if he can be held criminally responsible for the ambush murders of six young persons in the past year.

Officials said Berkowitz, 24, would spend at least the next two weeks locked in "spartan" quarters at Kings County Hospital in Brooklyn as psychiatrists begin probing his thoughts in conversations and tests. Guards watch him through a reinforced glass panel in a door.

"I'll try to engage the man in conversation, and as the conversation flows, that is how it will go," said Daniel Schwartz, the hospital's director of forensic psychiatry.

"It will be my function to ascertain whether or not he understands what is going on, if he is criminally responsible for his act . . . whether he can assist in his defense," Schwartz said, adding that the tests would take about two weeks - "perhaps a little longer."

Schwartz refused to say what, if any, discussion he has had with the accused killer.

Also today, Paul Billow, 25, a security officer at Saginaw, Mich., valley College who knew Berkowitz in the Army at Ft. Knox, Ky., said, "He used to tell us how heavily into dope he was.

"When I first met him in 1973 he was a Jesus freak," having renounced his adoptive parents' Judaism and having become a Baptist. "When he was outgoing and a source of laughs, he became sullen and reclusive. He didn't want to get involved in anything," Billow said.

He also said he believed Berkowitz had a "hangup" about women, particularly those with dark hair, because of his Army experienced in Korea.

"He'd talk about how liberal the Korean girls were about sex and that," Billow said. "He'd tell us how they'd just come up to him on the streets and proposition him. I think all but the last of his victims had black hair. Maybe these girls taunted him somehow."

Meanwhile, federal agents in Houston said Thursday that the five-shot, .44-cal. Charter Arms Bulldog pistol Berkowitz allegedly used in the murders was purchased June 12, 1976, by Billy Dan Parker of Houston at the city's Spring Branch Jewelry and Loan Co.

"This is the gun," said W.M. Rothgeb, agent-in-charge of the Bureau of Alcohol, Tobacco and Firearms office in Houston.

Rothgeb said he has not talked to Parker, a construction worker who was Berkowitz's best Army buddy, and did not know how the gun was transferred from Parker to Berkowitz.

"We have nothing at this point to show that anything illegal has been done," Rothgeb said.

Parker was unavailable for comment. But a Houston television station reported that he said in an interview that he bought the gun for Berkowitz, who was in Houston at the time, because Berkowitz said he wanted the weapon for protection on his drive back to New York.

Berkowitz "said he was afraid of something," KHOU-TV quoted Parker as saying.

Berkowitz reportedly asked Parker to buy the gun because he did not have a Texas driver's license, required for identification in weapons purchases. Parker said he used Berkowitz' money for the purchase.

In Albany, New York, Gov. Hugh L. Carey's office announced today that he had signed into law a bill requiring that profits from anything that individuals convicted of crimes do after the fact to make money from their notoriety be kept in a special fund available to the victims.

Attorneys for Robert Violante, 20, the last person wounded in the series of shootings, filed a $10 million damage suit against Berkowitz in Brooklyn Supreme Court Thursday.

The suit seeks $5 million punitive and $5 million compensatory damages for the shooting, which left Violante possibly blinded and killed his date, Stacy Moskowitz, 20.

The bill had been passed by the state legislature well before Berkowitz' arrest. Its sponsor, Sen. Emanuel Gold, a Democrat from Queens, said he introduced the bill because he thought whoever was arrested in the case might be offered large sums of money to tell his story in a book, magazine or newspaper or for movies or television.

Berkowitz, who is under 24-hour-a-day guard, was completely isolated today in the Kings County facility in a room large enough for three beds and a toilet. Two barred windows look out to the street and on another wing of the hospital, where Violante lies.

Berkowitz is allowed into the day room of the prison ward one hour each day, Schwartz said, either before or after the other inmates have been there, where he can watch television.

Meanwhile, a special Brooklyn grand jury convened for two hours today to consider an indictment against Berkowitz and the wounding of Violante.

The grand jury head several witnesses, including the arresting officers and the policeman who ticketed Berkowitz' illegally parked car not far from where Moskowitz and Violante were shot July 31. The jury will reconvene Monday, when it is expected to return and indictment.