There have been few reports about the changes in active and inactive renin throughout pregnancy. In this study, the changes in total and active renin concentrations during pregnancy were measured and their physiological roles are discussed. The present method employed radioimmunoassay of angiotensin I generated during incubation of plasma in an excess of sheep renin substrate. For determination of total renin concentration, inactive renin was activated by exposing plasma to trypsin. Concurrent with the progress of pregnancy, active renin increased gradually and reached its maximum during the third trimester, then fell sharply to first trimester levels by the fifth day of the puerperium. Total renin levels during pregnancy were about twice as high as puerperal levels. We found no difference in total renin levels between trimesters but a marked difference in individual women was observed. In the first trimester of pregnancy, inactive renin in plasma rapidly increased (to 4 times the average concentration in plasma of puerperal women), then declined slowly throughout pregnancy, and fell quickly after delivery. These results suggest that in pregnancy total renin levels are consistently elevated, 67% of which is inactive renin in the first trimester and active renin increases gradually with advancing gestation. After delivery, both active and inactive renin fall to normal levels within a few days.