A special grand jury, impaneled to investigate allegations of brutality and mistreatment of women prisoners in the Arlington County Jail, has broadened its focus and is investigating complaints that jail officials rely heavily on drugs to control prisoners, according to sources.

The special jury, which is expected to resume its third week of fact-finding today, heard testimony last week from jailers, former deputies and the jail's chief physician, Dr. Henry Horn, about the use of prescription drugs to calm inmates.

More than 53 percent of prescriptions written by Horn in February and March were for Valium, chloral hydrate and Dalmane, all used, according to medical sources, to relieve tension and stress.

Just under 30 percent of the jail's 137 inmates are taking these medications, according to Commonwealth's Attorney William S. Burroughs.

That number is slightly more than the average number of inmates in state institutions using similar medication, according to Wayne Farrar, spokesman for the Virginia Department of Corrections.

Horn has drawn criticism from deputies at the jail who alleged the physician is "too loose in his writing of prescriptions. It's like a code," said one deputy, who asked not to be identified. "The inmate will say, 'I have trouble sleeping," and the next thing you know, the doctor is writing out a prescription of Valium for it."

Horn, and Arlington County physician who has been the jail's doctor for seven years, denied the allegations.

"In general, a lay person would probably find any amount [of tranquilizers] more than normal," Horn said. "Prisoners are under tremendous emotional and mental stress and strain. Being incarcerated is the fourth most disturbing thing that can happen in your life."

Horn spends an average of two hours each Friday evening during the jail's sick call revewing the complaints of approximately 20 patients. Horn receives $10 for each inmate he sees and earns between $10,000 and $12,000 a year for these consultations, hospital visits and phone calls, acccording to Jeff Marin of the county's fiscal analysis division.

The jail has already exceeded its $55,000 annual mediccal budget for prescriptions and emergency treatment for inmates by more than $10,000, Marin said.

Lawyers for the Amrcian Civil Liberties Union in Virginia complained to the sheriff and to the commonsealth attorney's office in February about "drug prescriptions . . . used indiscriminately to control behavior," according to a letter signed by Janis L. McDonald of the ACLU.

Sheriff J. Elwood Clements and Burroughs inquired into the complaints and both said they found no basis for rfurther investigations.

Then unexpectedly last month, the ACLU attorneys took their case directly to the county's grand jury. A week later, that grand jury voted to give itself special investigative authority with subpoena powers, thus becoming the first special grand jury ever impaneled in Arlington.

According to sources, the special grand jury initially was investigating events surrounding an alleged incident of brutality against a woman inmate and general complaints about conditions in the women's section of the jail. The panel made specific inquiries about problems of access to lawyers and legal materials, the opening and reading of legal mail by deputies, improper monitoring of legal phone calls and lack of educational programs for women inmates, sources said.

But after a series of witnesses appeared before the jury, sources said, the focus of the investigation broadened.

One witness, Lucy Logan, an Arlington attorney, said she told the jury about an incident in which a woman client had been subjected to a strip search after being arrested on traffic charges by police.

Logan said the incident was "extremely upsetting" to her client, "I can't quarrel with a seach [in which an inamte is ordered to strip and her clothes and body are searched for contraband] in serious offenses. But this was for a minor traffic violation."

Testimony offered by Deputy Sheriff Ernie Buck, sources said, defended the jail procedure. Buck, who took a display case of articles seized during stip searches into the grand jury room, said later the sherrif's department must conduct the searches for the safety of not only the jail, but also the inmate.

"We've found loaded pistols, credit cards, knives, narcotics, firecrackers. And we never know who they'll come off of. Maybe someone who has a traffic violation against him here has committed a more serious crime elsewhere," Buck said.

Burroughs, who is acting as the special counsel to the jury, said it must issue a report on its finding within six months and could recommend indictments or changes in administrative procedures.

Clements, who has said repeatedly "the jail has nothing to hide," called the latest accusations "jailitis." If you don't give them enough [medicine], then they say it's not enough, and if you give them too much, that's wrong, too. The ACLU's done some good things, but they're on the wrong track here."