Plasma converting enzyme activity was measured by a spectre-photometric technique in plasma taken from normal mothers and fetuses at term during elective cesarean section or at spontaneous vaginal delivery. Plasma converting enzyme activity in the fetus was within the normal range for non-pregnant adults and for pregnant women undergoing spontaneous vaginal delivery. Fetal converting enzyme activity was the same whether delivery was accomplished by cesarean section or by the vaginal route, but maternal converting enzyme activity was significantly elevated at cesarean section compared with spontaneous delivery. These data are compatible with the thesis that fetal plasma converting enzyme is, at least in part, derived from extrapulmonary sources and that plasma conversion is not affected by the events that initiate labor or fetal respiration. The increment in maternal converting enzyme activity at cesarean section has not been adequately explained but may be due to the effects of anesthesia. Fetal plasma renin activity and angiotensin II concentration measured by radioimmunoassay at the time of vaginal delivery were markedly and proportionately elevated above simultaneous maternal levels. Maternalfetal differences were less prominent at cesarean section. The data suggest that the humoral events which initiate and maintain labor may stimulate the renin-angiotensin system in the fetus and that angiotensin I conversion in the fetus at term is adequate to maintain appropriate circulating angiotensin II levels. © 1978 by The Endocrine Society.