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There are two routes to supply oxygen to the myocardium of fishes, an oxygen-poor luminal supply and, in some fishes, an additional oxygen-rich coronary circulation. When present in teleost fishes, coronary vessels penetrate the myocardium to differing degrees. Elasmobranchs have coronary vessels throughout the myocardium. The different distributions of the coronary vessels reflect solutions to the problem of oxygen diffusion from the luminal blood to various parts of the heart, especially the outer layers of large ventricles. By examining cardiovascular data derived from in vivo experimental perturbations, such as exercise and environmental hypoxia, and from in vitro studies with perfused hearts, we discuss the extent to which luminal venous blood obviates the need for a coronary circulation and the extent to which the coronary circulation supplements the luminal oxygen supply. The oxygen content of the venous blood surpasses the maximal demand of the working heart and so the likely limitation of the luminal blood is the venous partial pressure of oxygen [Formula: see text] which in part determines the oxygen gradient to the mitochondria. The threshold venous [Formula: see text] is between 6 and 16 torr. Whether the coronary circulation is perfused in all species of fishes under resting conditions is not clear. Experimental data support the idea that there are species differences in the relative dependence on the coronary circulation. Coronary perfusion becomes relatively more important in situations where venous [Formula: see text] decreases and myocardial oxygen demand stays the same or increases, e.g., sustained aerobic exercise, environmental hypoxia, and the recovery period after exercise. Observations in vivo and in vitro support the idea that the pressure-generating potential of the heart, as opposed to the potential for flow output, is much more sensitive to changes in coronary blood flow. Coronary blood flow is more likely to be controlled through modulation of a large coronary vasodilatory reserve than through variations in perfusion pressure. Studies on coronary vasoactivity in fishes are limited; nonetheless, species differences are apparent.