Plasma levels of active and inactive renin were measured in the rat following manipulation of renin release. In the anesthetized rat intravenous captopril or sodium nitroprusside resulted in elevation of active renin only. Neither active nor inactive renin were altered following ether anesthesia of 2 hour bilaterally nephrectomized animals. Elevated levels of active and total renin were observed in anesthetized Wistar rats that had received oral captopril for a 6 week period. Renin substrate was decreased suggesting that blood angiotensin II exerts a tonic influence on renin substrate. In sodium deplete SHR, sacrificed by decapitation, chronic captopril resulted in a significant increase of both the active and inactive form of the enzyme concomitant with blood pressure reduction. The present data further confirm the independent control of active and inactive renin in the rat.