The effects of nicorandil on coronary collateral circulation during exercise-induced ischemia were compared between the different donor arteries in 13 patients with effort angina, 7 with complete obstruction of the left anterior descending artery (LAD) with well-developed collateral vessels from the right coronary artery (RCA) (LAD group), and 6 with complete occlusion of the RCA (segment 2-3) with well-developed collateral vessels from the LAD (RCA group). Initial percentage thallium (%TI) uptake (thallium-201 single photon emission computed tomography) and washout rate were measured in the anterior, septal and posterior regions during ergometer exercise. The submaximal treadmill exercise test was also performed using a cardiopulmonary monitoring system to measure Vo2 at anaerobic threshold (AT). After the controls were obtained, nicorandil (15 mg/day) was administered for 4 weeks, during which ergometer exercise and treadmill exercise tests were carried out repeatedly. A significant improvement of initial %TI uptake on exercise was observed in the LAD group with nicorandil therapy, but no improvement was shown in the RCA group. The AT significantly increased after nicorandil treatment in the LAD group (13.9 +/- 0.38-->16.8 +/- 1.18 ml/min/kg), reflecting the improvement of cardiac function through the increased collateral flow. However, in the RCA group, it remained unchanged, suggesting no improvement of cardiac function. Nicorandil was effective to increase collateral flow from the RCA, but ineffective on that from the LAD. Nicorandil is an effective coronary dilator and is reported to affect both large and small coronary arteries. The effect on the collateral circulation is dependent on the donor artery supplying different areas. The vasodilator effect of nicorandil is mainly on the LAD, which is large enough to supply blood to a wider area of the heart, rather than the RCA.