1. Administration of a single dose of captopril, 50 mg, to normal man (n = 7) on a low salt intake induced a 12.5 ± 0.9-fold rise in active renin. The rise in active renin was associated with a reciprocal decrease in circulating inactive renin to 10% or less of control levels. 2. Repeated administration of captopril (50 mg p.o. q 6 h × 3 days) to normal man resulted in increases in both active and inactive renin concentration in plasma. 3. When a single dose of captopril was administered to three patients with hyporeninaemic hypoaldosteronism, no changes in their circulating levels of active or inactive renin occurred. 4. These observations suggest that renal conversion of inactive to active renin may be important in active renin production.