The effects of succinylcholine and those of halothane on the action of pancuronium were studied in 40 patients. The degree of twitch depression and duration of action of pancuronium (0.02 rag/ kg) were greater in patients who received prior succinylcholine (1 mg/kg) than in those who did not. Halothane also increased the intensity and duration of action of pancuronium. Because of the pseudocholinesterase-inhibiting action of pancuroniam, the effect of succinylcholine given after pancuronium was also studied. There was no clinically significant increase in the effect of succinylcholine after pancuronium.