In 13 open chest dogs, the right atrium and various ventricular sites (i.e. the inflow and outflow tracts and apex of the right ventricle, apex and lateral wall of the left ventricle) were stimulated electrically at a given rate, ranging from 140 to 210 per min. Coronary circulation and cardiodynamics in response to ventricular pacing were compared with those to right atrial pacing at equivalent heart rate. Stimulation of the ventricle except lateral wall of the left ventricle produced no significant changes in coronary sinus blood flow and myocardial oxygen consumption despite decreased blood pressure and cardiac output. The minor change of coronary sinus blood flow was related to the decrease in coronary vascular resistance. Oxygen content of coronary sinus blood decreased during ventricular pacing, while arterial oxygen saturation remained constant. The response of coronary circulation to ventricular pacing did not differ essentially at faster and slower heart rates. It was considered that when stimuli were applied to the ventricular surface, the relatively constant myocardial oxygen consumption in the presence of lowered cardiac performance was attributed to the asynchrony of fractionate contractions of the ventricular muscle.