The isomyosin composition in left and right ventricles from aldosterone-salt-treated hypertensive rats and from vehicle-infused and aldosterone-infused normotensive control rats was compared. A significant incremental increase (20%) in the percentage of V1isomyosin and parallel decrease in the percentage of V3isomyosin occurred in both left and right ventricles from aldosterone-salt-treated animals compared with those in normotensive vehicle-infused controls. No change in the ventricular isomyosin distribution was observed in animals infused with aldosterone without salt, which indicates that aldosterone does not directly affect the ventricular isomyosin composition. The changes in left ventricular isomyosin composition were accompanied by significant left ventricular hypertrophy (38%;p< 0.05), whereas no hypertrophy was observed in the right ventricle. Plasma thyroxine levels were significantly lower in aldosterone-salt-treated rats (3.7 ± 0.6 μg/dl;p< 0.05) than in normotensive vehicle-infused (6.0 ± 0.7 μg/dl) or aldosterone-infused (6.7 ± 0.3 μg/dl) controls. These results indicate that factors such as alterations in thyroid status or a volume overload component of this hypertensive model, in addition to increased systolic blood pressure, may contribute to a biventricular shift in isomyosin cornposition in the aldosterone-salt model of hypertension.