To assess the influence of captopril on left ventricular mass in 24 normal Sprague-Dawley rats, 12 were given high sodium (group 1) and 12 low sodium (group 2) diets. Half the rats given each diet were treated with 30 mg.kg-1/day captopril by gavage, the others were given placebo. Mean(SEM) arterial pressure was significantly reduced in group 2 treated rats (102.3(2.0) vs 123.4(1.5) mmHg, p less than 0.0002) but not in group 1 treated rats (113.8(2.5) vs 123.7(2.9)mmHg, NS). Blood pressure response to a 200 ng.kg-1 iv dose of angiotensin I was blocked in both group 2 (8.3(2.1) increase vs 29.7(3.6) mmHg increase for controls) and group 1 treated rats (7.8(2.8) increase vs 36.5(4.0) mmHg increase for controls). In group 2 treated rats the left ventricular to body weight ratio (X 10(-3)) was reduced compared with control (2.1(0.05) vs 2.4(0.08), p = 0.026), whereas in group 1 rats this ratio was not significantly different in the treated and control groups (2.3(0.06) vs 2.5(0.18), NS), suggesting that the reduction in left ventricular mass resulted from the influence of captopril on blood pressure. It is concluded that captopril causes a reduction in left ventricular mass in normal rats as a result of a reduction in blood pressure, independent of the effects of angiotensin I converting enzyme. This supports the concept that left ventricular mass is determined primarily by wall stress and is capable of both upward and downward regulation.