Plasma cholinesterase (ChE), an enzyme produced by the liver, rapidly hydrolizes succinylcholine. In the presence of low enzymatic activity, the rate of destruction of succinylcholine is diminished and it may result in an undesirably long duration of paralysis following succinylcholine administration. There are many reports that a significant fall in plasma ChE activity was shown to occur during pregnancy and in the immediate postpartum period. The present study was designed to compare the plasma ChE activity and the reaction of succinylcholine in 27 healthy nonpregnant women with that in 30 pregnant women undergoing elective caesarean section. Before induction of anesthesia, venous blood was taken for the measurement of plasma ChE activity, which was assayed by a colorimetric method, and the duration of action, recovery index and TOF ratio of succinylcholine were measured using ABM (Anesthesia and Brain activity Monitor, Datex Co.). The results were as follows: 1. Plasma ChE aetivity in pregnant women was lower than in nonpregnant women but was not statistically significant. 2. With respect to the duration of action, recovery index and TOF ratio on the 25 and 75% recovery of first twitch height, there were no significant differences between pregnant and nonpregant women.