The effect of electrically induced auricular and ventricular tachycardia of various rates was studied in the anesthetized dog. When, the control heart ranging between 140 and 190 per minute, atrial tachycardia of a rate only slightly higher than the control rate was induced, a very temporary initial decrease in arterial blood pressure, cardiac output and coronary blood flow occurred, then all three parameters essentially returned to control level. With atrial tachycardia of a higher rate, blood pressure, cardiac output and coronary flow fell more markedly, then blood pressure and cardiac output rose to or toward control level, remaining below control level with higher rates of tachycardia, whereas the coronary flow rose to or above control level and only exceptionally remained below control level. Ventricular tachycardia had essentially the same effects as atrial tachycardia, but a ventricular tachycardia of a given rate had the same quantitative effect as an atrial tachycardia of a higher rate.