Most fish regulate cardiac output via changes in stroke volume whereas most mammals regulate cardiac output via changes in heart rate. We hypothesized that this change in mechanism of regulation would coincide with a change in the myocardial response to stretch. This possibility was tested in permeabilized cardiomyocytes prepared from rat and rainbow trout ventricles, where both sarcomere length (SL) and degree of Ca2+ activation could be controlled. Myofilament Ca2+ sensitivity activation was higher in trout than in rat at each SL tested (2.0, 2.3, 2.5 and 2.7 ìm). We also found that permeabilized trout myocytes produce greater passive tension at any given SL than rat. This was surprising as the trout heart is known to be more compliant. Interestingly, addition of phosphatase inhibitors in the permeabilizing solution reduced passive tension in the trout cells suggesting that phosphorylation of titin may be important in determining passive tension in particular in trout heart. In conclusion contractile properties in trout seem highly sensitive to the length-dependent modulation.