Twenty-six patients were given 3 mg isosorbide dinitrate either by direct intra-coronary injection (17 patients) or by intravenous injection ( patients), 5 minutes after an injection of 0.4 mg methylergometrine. In all 26 patients isosorbide dinitrate dilated the coronary arteries to a diameter that was 26% greater than the smallest diameter observed with methylergometrine and 14.8% greater than the basal diameter. The response was highly significant at 30 seconds, maximal at 2 minutes and lasted more than 10 minutes. At the dosage level used in this study, there was no significant difference between the two groups of patients. However, a rapid fall in systemic arterial pressure was noted after peripheral intravenous injection.