Rats with chronically implanted venous cannulas were trained to make differential active or passive shock-avoidance responses in a 2-compartment box at 15 sec after i.v. adminstration of pentobarbital (5 mg/kg) or saline. The discriminative pentobarbital stimulus persisted for several min after adminstration. The pentobarbital response also was elicited in tests at 15 sec after i.v. injection of chlordiazepoxide (2.5 and 5 mg/kg) or alcohol (500 mg/kg). Subsequent training at 10 min after i.p. injection of pentobarbital (10 mg/kg), followed by tests after i.p. administration of other drugs, indicated that the discriminative drug stimulus is qualitatively similar with this different route and time interval. A general preference for the passive over the active response developed only in the second stage of training, during i.p. injections.