Plasma renin activity and concentration and angiotensin II concentration have been measured serially in six normotensive primigravidae and in four mildly hypertensive patients in the four immediately following normal delivery. In normotensive patients, both renin and angiotensin II levels fell sharply over the first two hours to levels within the non-pregnant range, with a subsequent rise to levels, similar to those found in late pregnancy. Changes in the mildly hypertensive patients were smaller, and somewhat slower. It is suggested that the initial fall represents clearance of a renin of fetoplacental origin and that the subsequent rise may be a release from feedback suppression of renal renin synthesis.