After exposure of the hearts of eight dogs to a total of 45 min of myocardial ischaemia during cardiopulmonary bypass, mean left ventricular hydraulic output power (MLVP) decreased to an average of 49·5% of the original value and Vmax to 95·2% of its original value. During a constant intravenous infusion of isoproterenol, MLVP and Vmax expressed as a percentage of the depressed value noted after ischaemia, increased to an average of 229 and 164%, respectively. During epinephrine infusion MLVP and Vmax increased to an average of 171 and 123%, respectively, relative to the post-ischaemic levels. MLVP was shown to quantitate practically all of the mechanical output power of the left ventricle. Thus, if preload, afterload, and heart rate are controlled, MLVP should be representative of the contractile state of the left ventricle. Since changes of Vmax in response to positive and negative inotropic intervention did not correlate with MLVP, we concluded Vmax did not accurately reflect quantitative change of the contractile state of the left ventricle.