Cholinesterase activities were measured on serum from 941 pregnant women distributed evenly throughout the 40 weeks of gestation. A rapid fall has been shown to occur during the first trimester to a level which did not alter significantly during the remainder of pregnancy. Even lower values were observed in 105 patients examined during the first 7 days following delivery. Enzyme activity was used to determine the probable incidence of a prolonged reaction to suxamethonium in obstetric patients. The risk was greatest in the immediate post partum period affecting 11% of the patients investigated.