Heart rate was controlled by direct stimulation of the right ventricle of a dog previously subjected to complete A-V block in order to study the effect of this variable on cardiac output. Measurements of cardiac output were made at various heart rates in unanesthetized dogs, both at rest and walking on a treadmill. Except at the extremes, cardiac output was independent of heart rate at rest and during mild exercise. This finding emphasizes the fact that cardiac output may be regulated to provide required flow through changes in stroke volume alone, even though adjustment of heart rate is the usual means for obtaining this end.