The kinetics of the inhibition of human plasma cholinesterase (ChE) and erythrocyte acetylcholinesterase (AChE) by alcuronium, atracurium, d-tubocurarine, pancuronium, pipecuronium, and vecuronium were studied in blood drawn from 35 surgical patients.The activities of plasma ChE and erythrocyte AChE were determined by the calorimetric method of Ellman et al., using acetylthiocholine as the substrate. Lineweaver-Burk plots and Dixon plots were used for the analysis of the kinetics of both enzymes.The dissociation constants (K(m)) of plasma ChE and erythrocyte AChE were 5.00 x 10(-5) M and 5.28 x 10(-5) M, respectively, indicating that both enzymes have similar affinity to acetylthiocholine. Both Lineweaver-Burk plots and Dixon plots indicated that the six nondepolarizing neuromuscular blocking agents (NMBAs) at different concentrations induce linear mixed-type inhibition. The apparent inhibition constants (K(i)) of pancuronium (8.72 x 10(-8) M) and vecuronium (3.53 x 10(-7) M) for plasma ChE inhibition were lower than that of neostigmine (7.36 x 10(-7) M), whereas those of the six nondepolarizing NMBAs for erythrocyte AChE were markedly higher than that of neostigmine.Both plasma ChE and erythrocyte AChE were inhibited by six nondepolarizing NMBAs, and the pattern of inhibition of both enzymes was of mixed type. The inhibitory potencies of pancuronium and vecuronium for plasma ChE were larger than that of neostigmine, whereas those of the six nondepolarizing NMBAs for erythrocyte AChE were markedly lower than that of neostigmine. The rank order of relative potency for plasma ChE was pancuronium > vecuronium > pipecuronium > alcuronium > d-tubocurarine > atracurium.