The effects of endothelin-1 (ET-1), applied topically on the epicardium, on the coronary microcirculation of the beating canine heart were investigated in situ. ET-1 (1, 10, 100, and 1,000 pmol) induced a dose-dependent elevation of the ST segment in the epicardial ECG (n = 5). After application of ET-1 (100 pmol), the beating hearts were rapidly cross-sectioned and freeze-clamped in 120 ms using a specially developed device (n = 6). By NADH fluorescence photography of the cross-sectioned frozen sample, an increased fluorescent area in the subepicardium was clearly observable. The fluorescent dye injected from the left atrium was negative in the NADH fluorescent area. It can be concluded that ET-1, when administered extravascularly, induces severe vasoconstriction and myocardial ischemia in the microcirculation of the beating canine heart.