The function of the corpus luteum at midtrimester and at term was studied in 19 pregnant women. Six women had their pregnancies terminated by hysterotomy at 14-18 weeks. The remaining 13 women were delivered by cesarean section at term. Luteectomies were also performed on 6 women at the time of cesarean section. Progesterone and relaxin levels in peripheral blood serum were used as indices of luteal activity. Relaxin is a product of the human pregnancy corpus luteum. Relaxin secretion correlates well with luteal progesterone secretion. After cesarean section at term the decline in serum progesterone was parallel to that observed after hysterotomy at midtrimester, although the absolute levels were much higher at term. On the other hand, the actual relaxin concentrations, as well as the decrease in relaxin levels seen after evacuation of the uterus, were similar at midtrimester and at term. After luteectomy there was a more precipitous decrease in serum levels of both progesterone and relaxin than seen in nonluteectomized patients. Thus, in contrast to previous findings in lower primates, the corpus luteum of human beings does not regress in midpregnancy but, instead, remains functional throughout pregnancy. There is continued secretion of luteal progesterone and relaxin in the puerperium. The corpus luteum appears to be the only source of these hormones in the puerperium.