We investigated whether ventricular hypertrophy in reproductively mature male trout (Oncorhynchus mykiss) is associated with elevated hemodynamic loads. We measured ventral aortic blood pressure, pulse pressure dynamics, and blood volume in cannulated, unanesthetized trout with a wide range of relative ventricle masses (RVM, 0.076-0.199% of body wt). We also investigated in vitro pressure-volume dynamics in the bulbus arteriosus taken from trout with a wide range of RVMs. RVM was positively correlated with peak systolic pressure (SBP), mean blood pressure, and pulse pressure. Diastolic pressure and the absolute duration of arterial systole were similar among all animals, but a lower heart rate and a smaller relative duration of arterial systole were correlated with increasing RVM. Blood volume was expanded up to 34% as ventricles enlarged, and clearance of Evans blue dye was greater at higher SBP. Mass, maximal volume, and the pressure-volume dynamics of the bulbus were similar among all animals, suggesting that the bulbus did not compensate for ventricular enlargement. This conclusion was supported by the elevated maximal rates of arterial pressure development (+dP/dt) and decay (dP/dt) observed as RVM increased. We conclude that 1) mature trout are hypertensive and hypervolemic, 2) the dynamics of the bulbus may contribute to increased afterload, and 3) these changes in hemodynamic load may promote ventricular hypertrophy.