In open-chest dog experiments, 7 different types of cardiac failure were produced. In each type of failure, the effect of veno-arterial pumping and of left ventricular bypass (blood pumped from the left atrium to femoral artery) was investigated. Pressures were recorded in the left ventricle, the left atrium, the right ventricle, and the femoral artery.Veno-arterial pumping reversed failure of the right ventricle. It also was beneficial when overtransfusion had aggravated failure of the right or left ventricles, except in pre-existing aortic insufficiency. Veno-arterial pumping markedly increased decompensation of the left heart in the majority of observations, even though it occasionally reduced pressures in the left atrium and the right ventricle. The experiments suggest the use of veno-arterial pumping or of partial bypass with a heart-lung machine in acute cor pulmonale and in pulmonary congestion caused by abnormalities of the mitral valve where the left ventricle remains compensated. Veno-arterial pumping appears contraindicated in true failure of the left ventricle.Left ventricular bypass always restored compensation in all types of heart failure except cor pumonale. The experiments suggest the clinical use of left ventricular bypass in left heart failure.