The authors compared six heavy and six casual smokers of marijuana, using placebo, a fixed dose of marijuana, and a self-selected ad lib dose. The subjects showed a modest decrease in perception and psychomotor task performance with both types of marijuana dose, though casual smokers showed a greater degree of impairment than did heavy smokers. Subjectively, casual smokers experienced perceptual and affective responses to a placebo condition that mimicked their experience under marijuana. Some heavy smokers showed evidence suggestive of a mild hallucinogenic effect. The authors urge that all studies using smoked marijuana take note of the subjects' previous smoking experience and that these data be considered in the interpretation of findings on the effects of cannabis.