Plasma renin activity, the concentration of angiotensin I and the urinary excretion of aldosterone and sodium were studied longitudinally in 10 healthy primigravidae from the 10th week of pregnancy monthly until the 7th puerperal day. Plasma renin activity and angiotensing I levels were significantly elevated throughout pregnancy and showed maximal mean values at the 10th week, gradually returning to the level of nonpregnant women 7 days after delivery. They daily urinary excretion of aldosterone was also increased throughout pregnancy, but the mean values showed an increasing trend until 34th week. They returned to nonpregnant levels 7 days after delivery. There were no significant changes in the urinary excretion of sodium during pregnancy but the mean value was lower than in the nonpregnant control group. There was a significant positive correlation between urinary aldosterone and sodium values, while an almost significant negative correlation existed between urinary aldosterone and plasma renin activity, suggesting the priority of natriuresis in the regulation of aldosterone secretion during pregnancy. No corrleation was found between plasma renin activity and urinary sodium excretion values.