To determine the effects of captopril on left ventricular volumes and function in patients with and without depressed ventricular function following acute myocardial infarction (AMI) we studied 78 patients with a first Q wave AMI and no clinical evidence of heart failure. All patients underwent radionuclide ventriculography (RVG) on the 4th day after admission and were then randomly assigned to receive conventional treatment alone (36 patients, control group) or with the addition of oral captopril, 25 mg three times daily (42 patients, captopril group). RVG was repeated one month after the baseline examination. After one month the left ventricular ejection fraction (LVEF) significantly increased in the captopril group (from 43.2 +/- 1.3 to 50.9 +/- 1.6%, P < 0.001) and remained relatively unchanged in the control group (from 47 +/- 1.3 to 49.2 +/- 1.7%, P = ns). In the captopril group the subgroup of patients with a baseline LVEF < 45% demonstrated a significant decrease in end-systolic volume index (ESVI) (from 53.3 +/- 3.2 to 42.4 +/- 2.8 ml. m-2, P < 0.002) and a highly significant improvement in LVEF (from 36.3 +/- 1.3 to 49.6 +/- 1.8%, P < 0.00005). In the control group, LVEF also increased in those in whom it was < 45% (from 38 +/- 1.4 to 42 +/- 2.4%, P < 0.01), but the increase was less than that in the captopril group (P < 0.01), mainly due to an increase in end-diastolic volume index (EDVI) (from 78.2 +/- 4.6 to 84.6 +/- 12.3 ml.m-2, P = ns).(ABSTRACT TRUNCATED AT 250 WORDS)