The purpose of this study was to elucidate the mechanism that induces an improvement in exercise capacity by nitrates in patients with stable effort angina pectoris. The study population was composed of 19 patients: group A, 10 patients with chronic stable effort angina who had a well-developed coronary collateral circulation to the potentially ischemic region; group B, 9 patients with chronic stable effort angina who had no collateral circulation to the jeopardized myocardium. Treadmill exercise was performed according to the standard Bruce protocol with and without pretreatment with orally administered 10 mg isosorbide dinitrate. Percent increases (mean ± SE) in exercise duration were not significantly different between groups A and B (25 ± 6 vs. 14 ± 6%). Percent increases in the maximal rate-pressure product tended to be greater in group A than in group B (27 ± 6 vs. 10 ± 6%). Percent increases in the rate-pressure product at the onset of angina pectoris were significantly greater in group A than in group B (37 ± 7 vs. 7 ± 6%; p < 0.01). Percent increases in the rate-pressure product at 0.1 mV S-T segment depression were also significantly greater in group A than in group B (26 ± 6 vs. 1 ± 5%; p < 0.01). These results suggest that isosorbide dinitrate dilates epicardial collateral vessels with smooth muscle layers, but fails to dilate the coronary arteries with significant organic stenoses.