The physiological role of inactive renin, especially the question of whether and how a conversion to active renin takes place in vivo, remains controversial. In order to show the dynamic alterations from inactive to active renin following acute ACE-inhibition, both forms of renin were investigated in both renal veins and the peripheral circulation of 20 patients with essential hypertension and 20 patients with renovascular hypertension before and 1 h after 25 mg of captopril. Active and inactive renin were determined indirectly as plasma renin activity (PRA, unit: ng/ml x h). In vitro activation of inactive renin was achieved with trypsin (1 mg/ml plasma), followed by a further determination of PRA (= total renin). Subtraction of the active renin from the total renin yields the amount of inactive renin. In patients with essential hypertension, the mean values of active renin increase equally in both renal veins (1.4 and 1.3 before, 1.9 and 1.8 after captopril) and the peripheral circulation (0.9 and 1.3) (p less than 0.002), whereas the inactive renin decreases correspondingly. Renal veins: 7.6 and 8.2 before, 7.2 and 7.6 after captopril; peripheral circulation: 7.7 before and 7.0 after captopril (p less than 0.05). In all patients with renovascular hypertension, there is basally a marked lateralization of active renin (6.4 vs 3.5; p less than 0.01) and inactive renin (20.5 and 18.9, p less than 0.03) towards the side of the ischemic kidney. After captopril, the values for total renin and active renin increase (p less than 0.001), and the side difference for active renin becomes still more pronounced (33.0 vs 14.2; p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)