In sheep a gradual increase in the progesterone* level in peripheral plasma was observed during pregnancy. The maximum plasma levels were reached at about day 140, the mean value in two ewes, each carrying one lamb being 13.2 ng/ml, and in four twin pregnancies 20.0 ng/ml. During the last week of pregnancy a marked decline in plasma progesterone levels in all the intact animals was observed with a mean value of 2.1 ng/ml at parturition. Following ovariectomy the increase in plasma progesterone in advancing stages of pregnancy which has been observed in intact animals, was blocked. In the ovariectomized animals too a decrease in plasma progesterone levels prior to parturition was demonstrated.