An experimental model of acute myocardial infarction has been created in the rhesus monkey by surgical ligation of the anterior descending branch of the left coronary artery. The effect of verapamil on the size of myocardial infarct was investigated. Four groups of experiments were carried out. Group I animals served as operated controls. In group II, verapamil (Isoptin) was administered intravenously half an hour after coronary ligation and repeated every 6 hr for 24 hr. Group III animals were given verapamil orally in the dosage of 4 mg/kg, three times a day, 10 days before coronary ligation. In group IV, the chest was opened by the coronary artery was not ligated (sham-operated group). The infarct size was measured 5 days after coronary ligation by the histochemical technique of nitro-blue tetrazolium staining. The size of the infarct amounted to 22.05% +/- 1.34% of the left ventricular weight in group I, 19.40% +/- 2.08% in group II, and 14.08% +/- 1.26% (P < 0.01) in group III. No infarct was detected in group IV monkeys.