Some aspects of the control mechanism in the coronary circulation were investigated by using analog simulation. The simulation is designed to separate passive or mechanical factor from entire influence on the change in coronary flow consisting of passive and active factors, under different hemodynamic conditions. Simulated waveform was very similar to the blood flow pattern of the left circumflex artery actually obtained by an electromagnetic flowmeter. Pressure flow relationship was almost linear. Coronary flow increased with rise of aortic pressure, with the influence of active factors exceeding greatly that of the passive factors. When the heart rate was increased, the decrease in coronary flow due to the passive factors themselves was found. After the occlusion of the coronary artery the slight reactive hyperemia due to only passive factors was observed in the simulated flow.