Acute ischemic left ventricular failure was induced in anesthetized dogs by repeated coronary embolization with 50 microns microspheres. Plasma concentrations of atrial natriuretic factor (ANF) were measured by radioimmunoassay of arterial and venous samples before and after failure induction. Heart failure was accompanied by a doubling of arterial ANF concentration, whereas there were only insignificant changes on the venous side. The increase in arterial ANF correlated significantly to the increase in left ventricular end-diastolic pressure, but not to the increase in right atrial pressure. Measurements of pericardial pressure indicated that a pericardial constraint acted to reduce atrial distension and thereby cardiac ANF release during failure.