The nonhallucinogenic ergot derivative lisuride exerts many pharmacological effects that are similar to those of its hallucinogenic congener, lysergic acid diethylamide (LSD). Animals trained to discriminate between the presence of one drug and the other can be used to differentiate the actions of these compounds on a neuronal level. The discriminative stimulus effect of LSD (the LSD cue) is similar to that of the serotonin agonist quipazine, whereas the lisuride cue is similar to that of the dopamine agonist apomorphine. These data support the hypothesis that serotonin is intricately involved in the hallucinogenic effects of LSD.