Plasma renin activity was consistently elevated during the third trimester of pregnancy. The renin levels during labor were significantly elevated when compared to the third trimester values for the same women. No significant change occurred during parturition or for the first week post partum, indicating that neither the placenta nor fetus was the source of the elevated maternal plasma renin. By 6 weeks post partum, plasma renin had returned to normal. Comparison of renin activities in umbilical venous and arterial plasma demonstrated that the fetus (presumably the fetal kidneys) can produce renin and that the placenta is capable of adding or removing renin from the fetal blood.