Using Doppler echocardiography and electrocardiography, we characterized cardiac hemodynamics, timing, and electromechanical function, and examined the effects of ventricular hypertrophy on systolic function in anesthetized rainbow trout. Atrial filling (D(SA)), ventricular filling (D(AV)), and ventricular ejection (D(VB)) accounted for 40-77, 13-27, and 22-41% of the cardiac cycle, respectively. Ventricular ejection occurred entirely during atrial filling and ended by the time the QT interval was 80% (SD=9%) completed. Sinoatrial (SA) flow was of longer duration (0.53+/-0.08 sec, mean+/-SD) and lower velocity (32+/-8 cm sec(-1)) than corresponding atrioventricular (AV, 0.19+/-0.02 sec; 87+/-8 cm sec(-1)) and ventriculobulbar (VB, 0.30+/-0.05 sec; 63+/-20 cm sec(-1)) values. Despite a wide range of heart masses, atrioventricular and VB valve dimensions were identical ( approximately 5.5 mm(2)). Ventricle mass (M(V)), but not relative ventricle mass (RVM), and cardiac cycle length were positively correlated (r(2)=0.57, P<0.001); thus, all time-dependent electrical/mechanical measures of cardiac function were significantly related to M(V), but not RVM. All rate-corrected (c) electromechanical event durations (except cD(SA)) and the systolic function index (cPEP (pre-ejection period)/D(VB)) were independent of RVM, suggesting the maintenance of cardiac functional capabilities across maturation stages (males) and different ventricle sizes (males and females). In summary, we define fundamental electrical and mechanical properties of the in vivo teleost myocardium under anesthesia, and report the maintenance of systolic function over a wide range of heart sizes for both sexes and maturation state of males. We also suggest that the short duration of ventricular emptying relative to the QT interval may provide a novel mechanism to adjust stroke volume and cardiac output in teleosts.