The conflicting evidence regarding changes in serum cholinesterase during pregnancy is briefly reviewed. A technique for the estimation of serum cholinesterase by continuous titration at constant pH is described, and the results of a study of the enzyme in 149 pregnant women are presented. Pregnant women had a diminution of serum cholinesterase activity compared with non-pregnant women of the child-bearing age group, and this diminution is statistically highly significant. The fall appeared to begin soon after 10 weeks gestation. The reduction of enzyme activity in patients with toxaemia of pregnancy was not significantly different from that in non-toxaemic cases. Reasons are presented for believing that neither haemodilution nor hepatic insufficiency can provide an adequate explanation for the changes described. The possible clinical importance of the changes is outlined.