BACKGROUND: The reduction in the plasma cholinesterase (PChE) level results in slow to hydrolysis of succinylcholine (SCC) and mivacurium (MIV). The factors altering the level of the normal enzyme in human could be considered under the several conditions. We investigated in the present study whether the drugs induced decreases in normal PChE activity after administration of various muscle relaxants during anesthesia are evident and how these results should be influenced to the time course of neuromuscular blockade produced by SCC and MIV. METHODS: Young adult patients of ASA class I or II scheduled for elective surgery requiring muscle relaxation were premedicated and anesthesia was maintained with nitrous oxide in oxygen with increment of thiopentone or fentanyl as required. In the neuromuscular monitoring, surface electrodes were applied on the ulnar nerve at wrist. Supramaximal transcutaneous single twitch stimulation (1 Hz) during onset and 0.1 Hz during recovery of neuromuscular blockade induced by various muscle relaxants delivered by a peripheral nerve stimulator was applied. Twitch response of thumb adductor was measured mechanomyographically using 2 kg Load Cell Strain Gauge with thumb piece modification. Recordings were made on a Gould recorder. PChE levels were measured by the modified Garry method after induction of anesthesia and, at 3, 10, 20 and 30 min following administration of 2 x ED95 of pancuronium (PAN), vecuronium (VEC) and atracurium (ATR). Neuromuscular recordings were measured with onset time defined as lag time and manifest time, and recovery time defined as clinical duration, recovery index and total duration. RESULTS: The levels of PChE were significantly reduced after administration of PAN and VEC (p<0.05). Onset times were significantly shorten but recovery time in the group given MIV pretreated by small dose of PAN was significantly prolonged (p<0.05). And there were a evidence to prolong recovery time in the group pretreated by small dose of VEC but not significant. CONCLUSIONS: It is concluded that aminosteroidal derivative neuromuscular blocking agents have presumably evidence induced decreases in PChE activity rather than benzylisoquinolinium derivative neuromuscular blocking agents.