Background: The hydrolysis of mivacurium and succinylcholine is impaired in the presence of defects of pseudocholinesterase. Clinical reports are conflicting as to the utility of anticholinesterases, in the reversal of mivacurium- or succinylcholine- induced paralysis. In this study, the role of exogenous bovine pseudocholinesterases (BpChE) and/or neostigmine, pyridostigmine, edrophonium or galanthamine in the reversal of mivacurium- or succinylcholine-induced paralysis, were investigated with the rat phrenic nerve-diaphragm preparation. Methods: Ninety five Sprague-Dawley rats (200 g, male) were divided into 14 groups (n = 10). The phrenic nerve-diaphragm preparation mounted in a bath containing oxygenated Krebs' solution. Twitch response from diaphragmatic muscle evoked by phrenic nerve stimulation were measured. After stabilization of the twitch responses, mivacurium (0.1 ㎍/mlml) or succinylcholine (0.1 ㎍/ml) was administered incrementally in the preparation to obtain more than 95% twitch inhibition. BpChE (0.1, 1.0 u/ml), and/or neostigmine (0.1, 1.0 ㎍/ml), pyridostigmine (0.5, 5 ㎍/ml), edrophonium (0.01, 0.1 ㎍/ml) or galanthamine (0.1, 1.0 ㎍/ml) were added for the reversal of mivacurium- and/or succinylcholine-induced block in each group and the twitch responses (0.1 Hz) were monitored for 60 min. The effect of BpChE (0.1 u/ml), in combination with each of the above four anticholinesterases at lower concentrations also were examined. Twitch heights more than 75% was considered an adequate reversal. Results: BpChE 0.1 and 1.0 u/ml were effective in reversal of mivacurium-induced paralysis. When anticholinestrases were added, there was no effective improvement of twitch height at the end of 60 minutes. In succinylcholine-induced paralysis, BpChE was effective for reversal, but when anticholinesterases were added, BpChE potency was inhibited. Conclusions: BpChE will reverse mivacurium-induced block more effectively than anticholinesterase. BpChE is effective in reversing succinylcholine block. The addition of anticholinesterases inhibits the activity of pseudocholinesterase.