SUMMARYMale spontaneously hypertensive rats (SHR) and age matched Wistar Kyoto normoten-sive (WKY) rats of 5 weeks, 16 weeks, and 52 weeks of age were used to determine whether duration of hypertension has any effect on contractile protein ATPase and myosin isoenzyme distribution. Myofibrils, actomyosin, and myosin were isolated from the left ventricles of WKY rats and SHR and assayed for myosin ATPase activity and myosin isoenzyme distribution. Myofibrillar ATPase activity was assayed at various free [Ca⁺⁺] ranging from 10⁻⁷ to 10⁻⁵ mol·litre⁻¹. Ca⁺⁺ stimulated actomyosin ATPase activity was determined at several Ca⁺⁺ concentrations both at low ionic strength, which favours actin-myosin interaction, and at high ionic strength, which diminishes actin interaction with myosin. Purified myosin ATPase activity was assayed in the presence of K⁺-EDTA and in the presence of several concentrations of Ca⁺ ⁺. Actin activated myosin ATPase activity was assayed using 26 μmol·litre⁻¹ skeletal muscle actin. Under all these assay conditions no differences were observed in the contractile protein ATPase activity between SHR and WKY rats in any age group. On the other hand, in both SHR and WKY rats the contractile protein ATPase activity under all assay conditions was significantly decreased in 52 week old rats compared with 5 week old rats.The predominant myosin isoenzyme was V_i in 5 week and 16 week old WKY rats and SHR. In 52 week old WKY rats and SHR, however, significant amounts of isoenzymes V₂ and V₃ were present along with V₁. Percentage distribution of V₁, V₂, V₃ isoenzymes calculated from densitometric scans of gels did not show any differences between WKY rats and SHR in any age group. These results suggest that neither myosin ATPase activity nor myosin isoenzyme distribution is altered in the moderately hypertrophied left ventricles of SHR. Moreover, the data indicate that the myocardium of SHR, despite the persistence of pressure overload, undergoes a similar decrease in myosin ATPase activity and an increase in myosin isoenzyme V₃ to age matched normotensive WKY rats.