The harmful effect of tachycardia in patients with cardiac disease is well recognized. A sustained increase in rate is not infrequently an important precipitating cause of a break in compensation. Occasionally one observes symptoms of congestive failure occurring during an attack of paroxysmal auricular tachycardia in a person whose heart is normal in every other respect. Such a case has been reported by Barcroft, Bock and Roughton.1 However, some animals normally have heart rates which are considerably faster than those of patients with paroxysmal tachycardia. Why is a heart rate of 200 per minute likely to be attended by grave symptoms in men while the resting heart rates of guinea-pigs are more than 220? Again, why is it that the heart beats 600 times per minute in the mouse and 40 per minute in the horse (Clark2)? If the optimum (i.e., the normal) heart rate is different in