Two systems of sequential analyses on 12 ml aliquots of plasma have been developed for the quantitative estimation of the major forms in which estrone, estradiol and estriol occur in the circulation of pregnant women. The accuracy and specificity of these methods for the measurement of estrogen sulfates and glucosiduronates has been evaluated by recovery experiments in which 14C estrogens and authentic samples of estrogen conjugates, added to plasma, were subjected first to the hydrolytic procedures being investigated and then to the complete systems of analysis adopted. In the latter recovery experiments no less than 85% of the estrogen from any conjugate was recovered in the fraction specific for that conjugate, and no more than 2% in any other fraction. For the detection and measurement of any estrogen conjugated as both sulfate and glucosiduronate, the combined results from both systems of analysis on 12 ml aliquots of each specimen were required. Application of these 2 methods, to 4 pools of plasma from 8 women who were studied at the 36th to 38th week of normal pregnancy, indicated that at least f of the estradiol was free, that 70–83% of the estrone was in the form of estrone-3-sulfate, and that estriol was present in 3 conjugated forms: 35–46% as estriol glucosiduronate(s), 15–22% as estriol-3-sulfate, and 25–43% as a double conjugate, probably estriol-3-sulfate-16(17?)-glucosiduronate.