Background: Esmolol is rapid hydrolyzed by plasma esterase but may inhibit plasma cholinesterase activity based on its structure. This study was designed to evaluate the interactions between esmolol and succinylcholine or mivacurium which are metabolized by plasma cholinesterase and to determine the inhibitory effect of esmolol on human plasma cholinesterase. Methods: Neuromuscular effects of succinylcholine (1.0 mg/kg) and mivacurium (0.15 mg/kg) with or without esmolol (0.5 mg/kg or 1.0 mg/kg) were compared in 57 adult patients (ASA class I) during O2-N2O-isoflurane anesthesia. Neuromuscular block was monitored by recording the compound electromyogram of the hypothenar muscle resulting from supramaximal train of four stimuli applied to the ulnar nerve. Also plasma cholinesterase activity was measured before and 5, 10 minutes after injection of esmolol. Results: Time from injection to onset of over 95% block, clinical duration from injection to 25% recovery of control twitch, and recovery index defined as from 25% to 75% twitch recovery of succinylcholine or mivacurium were not altered by pretreatment of esmolol. Plasma cholinesterase activity was not decreased after injection of esmolol 0.5 mg/kg, but decreased by 5% after injection of 1.0 mg/kg (p<0.05). Conclusions: It is unlikely that neuromuscular blocking effects of succinylcholine and mivacurium are prolonged by administration of clinical doses of esmolol (0.5~1.0 mg/kg) due to inhibition of plasma cholinesterase activity in human.