The early (after 21 days) and late (after 4 months) effects of continuous treatment with captopril on left ventricular performance, weight and volumes were studied in rats with myocardial infarction. Early effects were examined in rats subjected to coronary ligation and randomized to either immediate treatment with captopril (2 g/liter drinking water) starting 2 h after surgery or no treatment. After 21 days, the treated group showed reductions in mean arterial, left ventricular systolic and left ventricular end-diastolic pressures compared with untreated rats. Right and left ventricular weight and left ventricular volumes were decreased and the ejection fraction index was increased by captopril treatment. To study the late effects of captopril, a second group of rats was randomized to immediate captopril treatment (starting 2 h after surgery), delayed captopril treatment (starting 21 days after surgery) or no treatment. When studied after 4 months of treatment, rats started on captopril treatment 2 h after infarction showed no differences compared with rats started on treatment 21 days after infarction. Treatment with captopril for 4 months produced changes that were similar to those in rats treated for 21 days. Captopril treatment improved hemodynamic function after myocardial infarction in rats examined after either 21 days or 4 months of treatment. The extent of the benefit was similar in the two treatment periods. Initiation of captopril therapy immediately after infarction did not appear to produce a greater effect than treatment started at 21 days after infarction in rats studied after the drug had been administered for 4 months.