Changes in blood pressure and heart rate in response to the renin inhibitor H77 and the angiotensin-converting enzyme inhibitor captopril were compared in conscious dogs during progressive sodium depletion. There were significant positive correlations between plasma renin activities immediately prior to administration of the inhibitors and the subsequent reductions in blood pressure produced by the inhibitors. The slopes and intercepts were similar for the two inhibitors, suggesting that both H77 and captopril were operating predominantly via inhibition of the renin-angiotensin system. Although the effects of H77 and captopril on heart rate and blood pressure were quantitatively similar, a small additional action of captopril was observed in dogs that had been sodium restricted for 12 days. Captopril injected during H77 infusion also had a small additional hypotensive action and caused a significant further increase in heart rate.