The hypotensive action of the angiotensin-converting enzyme (ACE) inhibitors enalapril and S-9490-3 was examined in conscious, chronically cannulated Na+-replete and Na+-deplete spontaneously hypertensive rats (SHR) of the Okamoto strain. Blood pressure, plasma ACE activity, plasma renin activity (PRA), and pressor responses to intravenous bolus injections of angiotensin I (AI) were measured over a 24-h period following a single oral dose of ACE inhibitor (0.3, 1.0, and 3.0 mg/kg) or vehicle. S-9490-3 caused a significantly greater hypotensive response and inhibition of plasma ACE and AI pressor responses than enalapril for each dose in both diet groups. Single oral doses of both drugs (3 mg/kg) caused slow, progressive falls in blood pressure which were maximal at 12 h. In contrast, inhibition of plasma ACE was maximal 1 h following the oral dose and returned to control levels over the 24-h period. The inhibition of the pressor response to intravenous AI paralleled, and was significantly correlated with, the inhibition of plasma ACE. There was no correlation between the maximal fall in blood pressure with PRA or with inhibition of plasma ACE activity in either diet group. The hypotensive response to both drugs at the 3-mg/kg dose was greater in Na+-deplete SHR than in Na+-replete animals. Both drugs caused large rises in PRA. The ACE inhibitor S-9490-3 is a significantly more potent hypotensive agent than enalapril in the SHR and a significantly more potent ACE inhibitor in vivo. The hypotensive response to both drugs was dissociated in onset and duration from the inhibition of plasma ACE and AI pressor responses.