A special grand jury investigating conditions at the Arlington County jail has criticized medical care, drug prescription procedures and supervision for inmates at the five-year-old facility.

The eight-member panel, whose findings were released yesterday by Circuit Court Judge Charles S.Russell, said it found no evidence of wrongdoing by jail personnel and recommended no indictments.

In its four page report, the special grand jury, the first ever impaneled in Arlington, attributed the problems at the jail to understaffing. Unlike regular grand juries, which can return indictments, special grand juries conduct investigations and report to a court.

The grand jury was originally impaneled to investigate allegations of brutality and medical mistreatment of female inmates and subsequently forcused on possible misuse of tranquilizers, including Valium and Dalmane, by the jail physician, Dr. Henry Horn.

Lawyers from the American Civil Liberties Union, whose testimony about medical care and treatment of inmates before an earlier grand jury led to the month-long probe, sharply criticized the report yesterday.

The finding of inadequate medical care is "vague to the point of being useless," said ACLU official Victor Glassberg. "I would have liked a clear finding on the propriety of medical care and the medical staff," said Glassberg, who characterized the report as being "couched in too general terms."

The panel, which met eight times, found that "some medical personnel have failed to respond in a reasonable length of time when deputies have requested emergency assistance."

Glassberg said that ACLU plans to file a class action suit against Arlington officials on behalf of jail inmates regarding what Glassberg called "the cavalier treatment of most medical complaints" by jail paramedics.

Built in 1974 at a cost $4.5 million, the starkly modernistic-and virtually windowless-jail in the county's courthouse complex is frequently touted by Arlington officials as one of the best in the state, a fact the grand jury noted in its report.

Among the jury's recommendations were increasing staff from 53 to 67 employes, improving control of prescription drug distribution, and the appointment of a special group to inspect and evaluate all phases of the jail and work speedily toward achieving accreditation of the jail by the American Correctional Association.

An official of that Washington based nonprofit association, formed 100 years ago to promote professionalism in corrections, said accreditation standards were developed only recently, and that only nine correctional facilities in the United States are in the process of being accredited.

J. Elwood Clements, due to retire shortly after 15 years as Arlington's sheriff, said he was generally pleased by the report. He said he thinks the jail can win accreditation "if we get enough personnel on board and the County Board allocates the funds."

County Board Chairman Dorothy T. Grotos said she thinks the report gave the jail "a relatively clean bill of health."

Grotos said she was unconvinced that the board should appoint a special group to inspect and evaluate the jail, as the panel recommended. "The jail is already inspected by a lot of outfit," she said. "I'm not sure we need another group."