The analgesic effects of delta 9-tetrahydrocannabinol (THC), the psychoactive component of marihuana, were tested using the formalin test. Rats were treated with either THC (5 mg/kg or 10 mg/kg) or a placebo by gavage 4 h before the formalin test for analgesia was initiated. THC produced a highly significant analgesic effect against both phasic pain and tonic pain. THC is discussed as a model for the development of new analgesics or as a suitable analgesic if used with another potentiating drug.