Most renin inhibitors are primate-specific. In the present paper, we describe the effects of CP-71,362, a pentapeptide which preferentially inhibits canine (and to a lesser extent, rat) plasma renin. Vs. the canine enzyme, its affinity (IC50 = 3.3 x 10(-12) M) is 1000x greater than for rat renin (IC50 = 3.3 x 10(-9) M), and 1000x greater than for human (IC50 = 2.3 x 10(-8) M), cynomolgus monkey (IC50 = 1.6 x 10(-8) M), or guinea pig (IC50 = 5.2 x 10(-8) M) enzyme. In anesthetized, sodium-depleted dogs, intravenous infusion of CP-71,362 (ED50 = 1.1 micrograms/kg/min) resulted in dose-dependent decreases (up to -35 mm Hg) in mean arterial pressure (MAP). The maximum fall in MAP was equivalent to that produced by i.v. captopril (5 mg/kg). Similar falls in MAP were observed in conscious sodium-depleted SHR (ED50 = 5 micrograms/kg/min). Via bolus injection, the action of CP-71,362 was relatively brief in dog, guinea pig, and SHR. We conclude that CP-71,362 is a potent canine/rat renin inhibitor and causes profound MAP lowering in these species.