A Virginia doctor, who pre-signed a prescription that was used by an assistant to dispense a drug, was convicted in D.C. Superior Court yesterday of aiding in the unauthorized practice of medicine.

The physician, Dr. Harvey Barry Jacobs, 35, operated the National Health Care Plan. In., a clinic that was at 36 N St. SE. He was charged with the offense, a misdemeanor, last December, after an undercover police officer visited the clinic and was given a prescription for a pain reliever by one of two physician's assistants who worked there.

There are no specific guidelines in the District to govern the conduct of physician's assistants, attorney's in the case said. As a result, the jury was instructed that they would have to decide whether the actions of Jacob's assistant were the generally accepted practice within the city's medical community.

However, during a week long trial before Judge Eugene N. Hamilton, the jury heard conflicting testimoney from expert witnesses about acceptable standards for physician's assistants.

No physician was at the clinic in November 1977 when Fernando Morales, a physician's assistant, gave the undercover police officer a prescription for the frug Darvocet-N, on a form pre-signed by Jacobs, according to testimony in the case. Morales was given immunity in exchange for his testimony in the case.

During the trial, Karl Katterjohn, director of the physician's assistants program at George Washington University, testified that it is not within accepted practice for a physician's assistant to fill out a pre-signed prescription.

Another expert, Noel McFarland, told the jury, however, that an assistant could write out a pre-signed prescription if it was reviewed by a physician within 46 to 72 hours. McFarland trains physician's assistants at Howard University.