To determine the neuromuscular effects of a new muscle relaxant, ORG NC45 (Norcuron), a monoquaternary homologue of pancuronium, 84 ASA [American Society of Anesthesiologists] Class I or II patients were studied under halothane and N2O anesthesia. The ED50 (dose of muscle relaxant causing a 50% depression of twitch tension) of pancuronium and ORG NC45 was 0.022 mg/kg (r = 0.90) and 0.015 mg/kg (r = 0.80), respectively, for a potency ratio of 1.5 (0.022/0.015). The duration of action (time from injection of 90% recovery of control twitch tension) was 27 .+-. 5 min with ORG NC45, 0.02 mg/kg, and 65 .+-. 16 min with pancuronium in an equivalent dose of 0.03 mg/kg. The increase in duration of neuromuscular blockade from repetitive doses was greater with pancuronium than with ORG NC45. Reversal of an ORG NC45 neuromuscular blockade was accomplished with doses of neostigmine slightly less than those required for pancuronium. Under thiopental-N2O anesthesia, endotracheal intubation was easily performed using ORG NC45, 0.07-0.14 mg/kg. The duration of action of ORG NC45, 0.07 mg/kg, was about 1/3 that of pancuronium (0.1 mg/kg). ORG NC45 is more potent and has a shorter duration of action with both initial and repetitive doses than does pancuronium. Further clinical trials are warranted.