Internal calorimetry was applied to the study of blood flow in the rabbit myocardium. In the normotensive animal resting flow per unit mass of tissue was about double that in the liver. Blood flow was a linear function of mean systemic blood pressure. Autoregulation was not a property of the myocardial vasculature, nor was there any trace of spasmodic closure of vessels at low pressures (critical closing). Under conditions of stabilized blood pressure, adrenaline and noradrenaline reduced myocardial blood flow. Acetylcholine, which in the present experiments did not slow the heart, produced a rise in myocardial blood flow when the blood pressure was stabilized.