Although it is well known that coronary inflow decreases substantially during systole, the mechanism responsible for this decrease remains controversial. Knowledge of how coronary input impedance is affected by contraction can differentiate between some of the proposed mechanisms. In open-chest dogs, we have measured coronary inflow in the beating heart both during constant-pressure perfusion and during 10-Hz sinusoidal pressure oscillations around the same constant pressure. By exploiting the principle of superposition, we have shown that coronary input impedance remains unchanged between systole and diastole. Using this result, we have shown that a simple lumped-parameter model with constant resistance and compliance can describe coronary inflow at heart rates of 60, 90, 120, and 150 beats/min both with vasomotor tone intact and during maximal coronary vasodilation. Coronary resistance and compliance determined using the model are comparable to those obtained in our laboratory and by others during normal diastoles and in the arrested heart. The results suggest that, despite large increases in myocardial tissue stresses during systole, coronary resistance and compliance as determined using inflow measurements are constant during systole and diastole.