Despite the widespread use of meperidine as an analgesic, its potential for producing delirium has been overlooked. Six cases demonstrating meperidine-induced behavioral toxicity are reported. Toxicity was more likely when meperidine was combined with cimetidine or drugs having anticholinergic activity. Discontinuation of meperidine and substitution of morphine for analgesia were usually successful in treating the delirium. Physostigmine reversed the delirium in one patient. The authors suggest that the delirium results from the excessive anticholinergic activity of meperidine or its only active metabolite, normeperidine.