In situ Starling and power output curves, and in vitro pressure-volume curves were determined for winter flounder hearts, and the hearts of two other teleosts (Atlantic salmon and cod). In situ maximum cardiac output was not different between the three species (~ 62 ml min^-1 kg^-1). However, because of the small size of the flounder heart, maximum stroke volume per ml g^-1 ventricle was significantly greater (2.3) as compared with cod (1.7) and salmon (1.4), and is the highest reported for teleosts. The maximum power output of the flounder heart (7.6 mW g^-1) was significantly lower than measured in the salmon (9.7) and similar to the cod (7.8), but was achieved at a much lower output pressure (4.9 vs. 8.0 and 6.2 kPa, respectively). Although the flounder heart could not perform resting levels of cardiac function at subambient pressures, it was much more sensitive to filling pressure; a finding supported by pressure-volume curves which showed that the flounder's heart chambers were more compliant. Finally, we report that the flounder's bulbus:ventricle mass ratio (0.59) was significantly higher than in the cod (0.37) and salmon (0.22). This data: supports previous studies suggesting that the flatfish cardiovascular system is a high volume, low pressure design; shows that vis-a-fronte filling is not important in flatfish, and that some fish can achieve high levels of cardiac output by vis-a-tergo filling alone; and suggests that a large compliant bulbus assists the flounder heart in delivering extremely large stroke volumes at pressures which do not become limiting.