The important rôle of the sulfhydryl group in metabolic processes has long been recognized. It is suggested the possibility that toxemias of pregnancy may be related to the sulfhydryl contents in sera. Therefore, the sulfhydryl activity in sera was measured in normal and toxemic pregnant women. The author has carried out experiments according to Brdicka's polarographic method on sera from 43 pregnant women. Summary 1 The free sulfhydryl group in sera was estimated by a Brdicka's method. 2 Among normal non-pregnant women, the sulfhydryl content expressed as a wave height on polarogram were nearly uniform. 3 Sulfhydryl activities of pregnant women in the last trimester show values about 21.8% lower than non-pregnant women. 4 Sera obtained from pregnant women with marked edema showed a significant reduction in the sulfhydrylcontent and markedly in women with eclampsia. 5 The application of serial serum sulfhydryl determinations in pregnant women may be of value for study on toxemias of pregnancy.