The intravenous administration of sodium pentobarbital to dogs is associated with a decrease in cardiac contractile force and arterial blood pressure. The latter appears to be due to vasodilatation in major vascular beds, rather than primarily to cardiac depression. Indeed, in most animals cardiac output increases, presumably as a function of increased venous return. The regional blood flows are frequently maintained or augmented, in spite of the decreased arterial perfusion pressure. When regional flows do decrease, the operative mechanism appears to be the decreased arterial perfusion pressure, rather than reflex vasoconstriction.