The muscle relaxant effects of ORG NC 45 and pancuronium were compared in anesthetized patients with normal liver and renal functions. In all patients, the muscle relaxant effect was monitored by measuring the strength of the adductor pollicis muscle elicited by supramaximal stimulation of the ulnar nerve at the wrist. Different modes of administration of the muscle relaxants were used. In order to facilitate tracheal intubation, a dose of 100 mu X kg-1 of ORG NC 45 or pancuronium was administered. This dose was followed or not according to the type of surgery by repeated doses of 25 micrograms X kg-1. The results of this study indicate that ORG NC 45 is much shorter acting than pancuronium, and non cumulative. The dose of 100 micrograms X kg-1 produced adequate conditions for tracheal intubation 3 to 4 min after the administration of the muscle relaxant. The recovery of the muscle strength to 75% of control value was achieved 46 min and 100 min after a single dose of 100 micrograms X kg-1 of ORG NC 45 and pancuronium respectively. The interval of time between repeat injections averaged 20 min for ORG NC 45 and 40 min for pancuronium.