The effects of 75 mg./sq.m. of ketamine intravenously (I.V.) on neuromuscular blockade with d-tubocurarine, pancuronium bromide, or succinylcholine were studied in 38 surgical patients anesthetized with 1 percent halothane, and compared with those obtained with halothane under identical conditions but without ketamine. Neuromuscular blockade was quantitated by force of thumb adduction in response to supramaximal stimulus of the ulnar nerve. Ketamine alone resulted in an increase in twitch height of 2 to 10 percent in 23 of 38 patients. Also, arterial blood pressure decreased a mean of 22 ± 2.8 (S.E.) torr following ketamine. No change was observed in the median effective dose of relaxant (ED50) needed to produce 50 percent depression of twitch height with ketamine in patients receiving succinylcholine or pancuronium. The ED50 of d-tubocurarine was decreased from 4.9 mg./sq.m. with halothane to 2.8 mg./sq.m. when ketamine was added to the halothane anesthesia. Venous blood concentrations of ketamine were the same in the three relaxant groups. The results of this study allow only speculation concerning the mechanisms of the increase in twitch height observed with ketamine and the potentiation of d-tubocurarine but not pancuronium.