Renin activity and the concentrations of angiotensin I and angiotensin II in amniotic fluid of second- and third-trimester pregnancies were determined by radioimmunoassay. Between the 28th and 38th wk of gestation, the mean renin activity in the amniotic fluid was higher than during early pregnancy (before the 18th wk of gestation). Both renin activity and the concentrations of angiotensin I and II were increased on some cases of Rh-incompatibility. One to two weeks after the administration of betamethasone to the mother with threatened premature delivery, the intra-amniotic renin--angiotensin system was slightly suppressed. In urine samples of newborns, angiotensin concentrations were in the same range as those found in the amniotic fluid; renin activity was very low or undetectable in the urine of male neonates (1--7 days of age). Thus, angiotensin II in the amniotic fluid may be derived both from fetal urine and/or as the product of enzymatic reactions in the amniotic sac; the latter is dependent not only on the presence of renin and converting enzyme but also on the local renin substrate (angiotensinogen) concentration.