It is known from experiments that angiotensin-converting enzyme inhibitors can limit infarct size. In a prospective, randomized, placebo-controlled double-blind study, 22 patients were given 1.5-2.0 mg captopril/h i.v., while 24 patients were given placebo. Medication was started between 2 and 18 h from the onset of infarction. The two groups were matched for age, infarct location, and time of intervention. With the exception of one patient in either group, all were concurrently given nitroglycerin. The necrosis parameters were provided by the quantitative measurement of the QRS complex. The Q wave decreased with captopril treatment (-0.003 mV), but increased with placebo (+0.14 mV, p < 0.05). The number of ventricular premature beats at 24 h from the start of treatment was 25/h with placebo, and 9/h with captopril (p < 0.02). Ventricular fibrillation occurred seven times in the placebo group, but did not occur in the captopril group. The creatine kinase infarct weight was 59 gram-equivalents (gEq) with placebo, and 45 gEq with captopril (p = NS). Mean arterial pressure was reduced by 12 mmHg with captopril treatment. The results show a beneficial effect of captopril on infarct size and electrical instability, over and above the effect of standard management with nitroglycerin and thrombolysis.