We tested the hypothesis that the feline left ventricle normally works at optimal external power as opposed to optimal efficiency by (re)analyzing data from five isolated, blood-perfused cat hearts and 39 open-thorax cats. In the isolated hearts, we measured pump function, external steady power, myocardial oxygen consumption, and efficiency. Optimal external power and optimal efficiency were found at different left ventricular outputs (6.94 +/- 0.33 and 8.35 +/- 0.37 ml/s, respectively; P less than 0.001). In the in situ cat hearts the working point was found at an output of 4.72 +/- 0.32 ml/s, whereas optimal external power was found at 4.84 +/- 0.26 ml/s. These values were not significantly different. Assuming that the point of optimal efficiency was located at the same fraction of the maximal unloaded left ventricular output (Fmax) as in the isolated hearts, i.e., 0.7, we found the point of optimal efficiency for the in situ heart at a flow of 5.83 +/- 0.32 ml/s, which was significantly different (P less than 0.001) from the flow in the working point. Our data therefore indicate that the left ventricle in the open-thorax cat is matched to the arterial load such that its external power output rather than efficiency is optimized.