Four chimpanzees were trained to respond on three operant schedules of positive reinforcement. Amounts of a marihuana extract containing from 200 to 800 µg/kg (–)-Δ9-trans-tetrahydrocannabinol (Δ9-THC) were orally administered 2.5 h prior to experimentation. Although the separate reinforcement schedules were differentially sensitive to the drug, significant behavioral effects were produced by the smallest Δ9-THC doses used. It was concluded that the behavioral effects of Δ9-THC, at oral doses within the effective minimum range for man, can be adequately assessed in chimpanzees and that the chimpanzee may be a suitable substitute for man in the study of long-term effects of marihuana.