Pentazocine is a new nonaddicting analgesic derived from the benzomorphan nucleus. A preliminary single-blind study carried out postoperatively on 185 male patients was performed to determine analgesic effects and side actions, especially concerning respiratory function. Analgesia was recorded within 20 minutes of intramuscular injection of 20-, 30-, and 40-mg doses, and this persisted for three hours at an optimal level. Side effects noted most frequently were drowsiness, mild respiratory depression, and moderate deviations in blood pressure, but none of these was considered hazardous. The absence in pentazocine of nalophine hydrochloride-like psychic effects and morphine-like addiction suggests that this substance may be a valuable aid for the control of severe pain.