Eight rats were trained to barpress for water reinforcement under a variable interval 60-sec operant schedule. Nine acute oral administrations of (–)Δ9-trans-tetrahydrocannabinol (Δ9-THC), in amounts ranging from 0.25 to 64 mg/kg, produced dose-related effects on responding; overall response rate increased at lower doses, while higher doses produced a suppression of responding and a corresponding loss of reinforcements. It was concluded that oral Δ9 -THC has a biphasic effect on operant behavior in rats which closely resembles that obtained with intraperitoneal drug administrations.