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The time-course of action of 0.1 mg/kg vecuronium and 0.5 mg/kg atracurium was investigated in nonpregnant and in postpartum patients within 4 days after delivery. The clinical duration of vecuronium, but not that of atracurium, was significantly prolonged in the latter group (P < 0.001) and averaged 36 ± 6 and 37 ± 4 min for atracurium and 32 ± 6 and 49 ± 10 min for vecuronium in nonpregnant and in postpartum patients, respectively (mean ± SD). In additional in vitro experiments in the rat phrenic nerve-hemidiaphragm preparation no difference could be observed in the neuromuscular blocking effects of vecuronium in postpartum and nonpregnant female rats. It is concluded that pregnancy-induced changes in liver blood flow and/or competition for the liver uptake of sexual hormones might interfere with the hepatic clearance of vecuronium in postpartum patients and thereby cause the observed Prolongation of neromuscular blockade.