1. Homologous angiotensin-I was added to untreated plasma. Angiotensin-II which is formed by plasma converting enzyme and subsequently degradated by angiotensinases was determined as a function of time by using radioimmunoassay. Having determined the kinetics, the activities of converting enzyme and angiotensinases were calculated by a least-square fit of the theoretical curve to the experimentally measured values.
2. The method gives a simple measurement of converting-enzyme activity in untreated plasma; in plasma from salt-depleted and renal hypertensive rats this was found to be slightly increased but was normal in plasma from nephrectomized rats.
3. The half-lives for angiotensin-I in normal rat and human plasma were found to vary between 0·8 and 2·1 min, and the possibility that plasma converting enzyme participates in the regulation of the formation of angiotensin-II in vivo cannot be excluded.
4. The angiotensinase activity in rat plasma gave half-lives for angiotensin-II between 1·5 and 3·3 min; the half-life in normal human plasma was 10 min.