A 70-year-old Northwest Washington physician was convicted in U.S. District Court yesterday of illegally prescribing appetite-supressing drugs to an undercover policewoman who told him she was selling the drugs to friends.

Dr. John G. Todd, whose practice is located at 8 Rhode Island Ave. NW, faces possible revocation of his medical license as well as a maximum eight years in prison and $25,000 in fines as a result of the conviction on two counts of distributing federally controlled substances.

Todd had testified that he prescribed the drugs--mazindol and diethylproprion hydrochloride, marketed under the trade names Sanorex and Tepanil--because the 114-pound policewoman, posing as a patient, complained of being unable to control her appetite.

Jurors heard prosecution testimony, however, that the drugs are to be used only to control obesity. Prosecutor Douglas Behr maintained that Todd never examined the officer for a weight problem and prescribed the drugs whenever she asked, in return for a $10 fee.

Jurors also heard secretly recorded conversations in which the policewoman told the doctor she needed additional drugs to sell to friends. In one tape, Todd told her he would give her a prescription under a different name and suggested she take it to a different drug store in order to avoid arousing suspicion.