Iva-His-Pro-Phe-His-Sta-Leu-Phe-NH2 is a new, potent, specific statine -containing renin inhibitor. In vitro, the ID50 needed to inhibit both dog and human plasma renin is approximately 10(-8)M. Injections into anesthetized rats receiving a continuous intravenous infusion of hog renin revealed a dose-related lowering of mean arterial blood pressure (MAP) with an ID50 of 0.04 mg/kg. In conscious Na-deficient dogs, infusion of the inhibitor for 48 hours resulted in a sustained lowering of MAP and suppression of plasma renin activity (PRA). To study the relationship between MAP lowering and inhibition of PRA, conscious Na-deficient dogs received continuous intravenous infusions for 1 to 3 hours. At doses of 20, 40, and 160 micrograms/kg X min, MAP was reduced 9 +/- 3, 15 +/- 0, and 22 +/- 4 mm Hg. No dose-related response was observed for PRA, which decreased from 7.8 +/- 0.9 to 0.4 +/- 0.3, 0.1 +/- 0.1, and 0.4 +/- 0.2 ng angiotensin I/ml X hr, respectively. Further studies using much-reduced infusion rates revealed a dose-related inhibition of PRA but not MAP. Doses of 0.1, 0.2, and 1.0 micrograms/kg X min inhibited PRA, 34% +/- 1%, 52% +/- 3%, and 82% +/- 4% while MAP decreased, 3 +/- 1, 4 +/- 1, and 2 +/- 1 mm Hg, respectively. These data reveal the potent blood-pressure-lowering effects of this new renin inhibitor and suggest that there may not be a direct relationship between inhibition of circulating renin and blood pressure lowering in Na-deficient dogs.