BACKGREOUND: The magnitude of neuromuscular blockade is related to plasma concentration of muscle relaxants. This study was designed to compare the maximal depression of twitch height by blood flow occlusion using a tourniquet at various time interval after intravenous administration of muscle relaxants. METHOD: We studied 127 healthy male adult patients who underwent elective surgery under the general anesthesia with propofol infusion and 50% nitrous oxide. The single supramaximal twitch stimulation applied to the ulnar nerve at the wrist at 1 Hz. The twitch response of adductor pollicis muscles were measured by a 2 kg Load Cell strain gauge with a thumb piece modification and recorded by a Gould TA 240 recorder. After occlusion of blood flow by the tourniquet in the upper arm, in which the neuromuscular monitoring was applied on the wrist, we administered the equipotent dose (ED95) of succinylcholine (S group), mivacurium (M group), and vecuronium (V group) intravenously on the contralateral arm respectively. We measured the maximal depression (%) of twitch height after the releasing tourniquet at 30, 60, 90, 120, 150, and 240 second intervals after the injection of each drug. RESULTS: The depression of twitch height was not found from 90 seconds of tourniquet time in the M group, and 120 seconds of tourniquet time in the S group. However, in the V group, the depression of twitch height was maintained to 240 seconds of tourniquet time. CONCLUSIONS: It is suggested that the plasma concentration of mivacurium declined faster than that of succinylcholine, and that of vecuronium decreased slowest among the groups after intravenous administration of equipotent dose (ED95).