The inhibitor of human renin, H142, was studied in nine male volunteers. On three occasions, in random order, volunteers were infused with 5% dextrose or with H142 at 1.0 or 2.5 mg/kg per h for 30 min while supine and thereafter with dextrose for 1½ h. There was a marked reduction in plasma active renin concentration as assayed by an enzyme-kinetic method, with parallel falls in the circulating concentrations of angiotensins (ANG) I and II, all of which rebounded transiently to values above basal after the H142 infusion was stopped. In contrast, total renin concentration as measured by radio-immunoassay rose while ANG I and II fell, subsiding when H142 was discontinued. There was a slight but significant increase in plasma noradrenaline as renin became inhibited; plasma adrenaline was unchanged. H142 produced a slight fall in systolic blood pressure (SBP) and a clearer, highly significant, dose-related fall in diastolic blood pressure (DBP). There was a modest but significant increase in the heart rate. These studies confirm H142 as an effective inhibitor of human renin in vivo.