In the mammalian heart the metabolic costs of pressure loading exceed those of volume loading. As evidence suggests that the opposite may be true in fish, we evaluated the metabolic costs of volume and pressure loading in the isolated trout heart and compared the results with the mammalian heart based on the biomechanical properties of cardiac muscle. The highest power output (2.33+/-0.32 mW g(-1), n=5) appeared at the highest preload pressure tested (0.3 kPa) and at an afterload of 5 kPa. At a higher afterload, power did not increase because stroke volume fell. The highest mechanical efficiency (20.7+/-2.0%, n=5) was obtained at a preload of 0.15 kPa and an afterload of 5 kPa. Further increases in preload or afterload did not increase mechanical efficiency, probably because of increases in ventricular wall stress which increased the oxygen consumed disproportionately more than the stroke work. Under pressure unloading (25% decrease in power output), mechanical efficiency was significantly higher in comparison with volume unloading. Given that stiffness of the ventricular tissue is larger in trout than in rat papillary muscles, it is suggested that the increased strain during volume loading is energetically disadvantageous for stiff muscles like those of trout, but it is advantageous when muscle stiffness is lower as it occurs in the rat papillary muscle.