In an attempt to increase the selectivity of drug discrimination, rats were trained to discriminate LSD (0.08 mg/kg) from a group of "other" compounds consisting of cocaine (10 mg/kg), pentobarbital (5 mg/kg), and saline. Acquisition of this LSD-other discrimination was rapid (31 days) in chambers equipped with retractable levers and did not differ significantly from that of a group of animals...