We examined the effects of ischemia with and without reperfusion on endothelium-dependent and -independent vascular relaxation in both conduit and resistance coronary arteries. Studies were performed on dogs under control conditions (n = 13) or after 1 hour of circumflex coronary artery occlusion with (n = 10) or without (n = 8) 1 hour of reperfusion. Rings of obtuse marginal branches of the left circumflex coronary artery (conduit arteries) were studied in organ chambers. Coronary microvessels (110-220-microns diameter) were studied in a pressurized state with an in vitro microvessel imaging apparatus. Relaxation was evaluated after preconstriction with prostaglandin F2 alpha and U46619 (a thromboxane A2 analogue) in conduit and resistance vessels, respectively. Conduit vessel function was not altered by ischemia with or without reperfusion. Endothelium-dependent microvascular relaxation was depressed in response to acetylcholine, ADP, and calcium ionophore A23187 after ischemia with reperfusion compared with control relaxation (ED50 as -log[M]: 6.0 +/- 0.2 [p less than 0.05], 5.1 +/- 0.4 [p less than 0.05], and 5.8 +/- 0.1 versus 6.8 +/- 0.2, 6.8 +/- 0.2, and 6.6 +/- 0.2, respectively). Ischemia without reperfusion modestly altered microvascular endothelium-dependent relaxation. Microvascular relaxation to nitroglycerin was not altered by ischemia with reperfusion. We conclude that 1) endothelium-dependent relaxation in large epicardial coronary arteries is relatively refractory to ischemia with or without reperfusion, 2) ischemia alone produces mild alterations of coronary microvascular reactivity, 3) ischemia followed by reperfusion produces a marked and selective impairment of endothelium-dependent responses in the coronary microcirculation.