Atherosclerosis impairs endothelium-dependent relaxation of large conduit arteries. Because coronary resistance vessels are spared from the development of overt atherosclerosis, endothelium-dependent responses were examined in these vascular segments. Malaysian cynomolgus monkeys (n = 6) were made atherosclerotic by being fed a 0.7% cholesterol diet for 18 months. Control monkeys (n = 6) were fed a standard diet. Coronary microvessels (122-220 microns) were studied in a pressurized (20 mm Hg), no-flow state using a video-imaging apparatus. Relaxations of microvessels, preconstricted with the thromboxane analogue U46619, were determined in response to acetylcholine, bradykinin, the calcium ionophore A23187, adenosine, and sodium nitroprusside. Microvascular relaxations to bradykinin and A23187 were reduced in atherosclerotic monkeys compared with controls, whereas acetylcholine produced additional contraction in atherosclerotic monkeys. Responses of preconstricted microvessels to adenosine and sodium nitroprusside were identical in atherosclerotic and control animals. Indomethacin did not alter responses in control or atherosclerotic animals. Histologic examination revealed neither intimal thickening nor plaque formation in microvessels of this size class despite marked changes in conduit arteries. Electron microscopy showed minor alterations of endothelial cell morphology in microvessels of atherosclerotic animals. In conclusion, long-term hypercholesterolemia markedly impairs endothelium-dependent vascular relaxation in the coronary microcirculation where overt atherosclerosis does not develop. These changes in endothelial cell function may significantly alter regulation of myocardial perfusion by neurohumoral stimuli.