Many attempts to obtain reliable self-administration behavior by laboratory animals with delta-9-tetrahydrocannabinol (THC), the psychoactive ingredient in marijuana, have been unsuccessful. Because self-administration behavior has been demonstrated in laboratory animals for almost all other psychoactive drugs abused by humans, as well as for nicotine, the psychoactive ingredient in tobacco, these studies would seem to indicate that marijuana has less potential for abuse. Here we show persistent intravenous self-administration behavior by monkeys for doses of THC lower than doses used in previous studies, but comparable to doses in marijuana smoke inhaled by humans.