Because of the clinical implications, there has been an active search in recent years for a link in heart muscle between biochemical and mechanical parameters. Barany’s observation [Z] of a positive correlation in different types of skeletal muscles between intrinsic, actin-stimulated myosin adenosine-triphosphatase activity (ATPase) and muscle maximum speed of shortening (V max) raised the question of a similar relation in cardiac tissue. The major problem in approaching this question in heart muscle was the technical difficulty in determining myosin ATPase activity in the structures best suited for mechanical studies i? z vitro, ie trabeculae or papillary muscles of small size. By assuming that the ventricular cavity was homogeneous, a relationship between ATPase activity and V,,, has been suggested by comparison of data obtained separately on different myocardial fragments for normal hearts of different animal species and in various states of myocardial impairment {for review see [1.5]). Recent findings have shown that rat heart ventricles contain two major isoenzymes, V, and V,, which exhibit very different ATPase activities, and whose ratio can change drastically in different physiological and pathological conditions [4, 6, 8, 9, IQ, 131. Electrophoresis in non-dissociating conditions allows the determination of myosin isoenzymic distribution on minute amounts of tissue [I, S]. We demonstrate here, by comparing normal and various pathological states such as cardiac hypertrophy and hypophysectomy, that myosin isoenzymic composition and maximal speed of shortening of papillary muscle are linearly correlated. These results constitute direct evidence that cardiac contractility{defined as the time independent interrelationship between force, velocity and length [3]) is determined by myosin composition, independent of the etiology of myocardial impairment. The experimental models selected for this study were those in which changes in contractility and myosin composition have been previously described: old control animals, cardiac hypertrophy induced by abdominal aortic constriction and hypophysectomized animals {for review, see [IO, 151). S ex and age-matched unoperated rats of the same strain, kept under identical housing conditions, were also used. Maximum unloaded shortening velocity of isolated papillary muscles was determined by the zero load clamp technique [3]. With this technique, V,,, is measured directly and this measured value expressed as 1 max/s, characterizes the force-velocity-length relationship which is independent of time and initial length. Myosin isoenzymic composition was determined by electrophoresis of crude extracts of the papillary muscles under non-dissociating conditions employing pyrophosphate as a buffer to solubilize myosin [I, 61. Rat cardiac myosin is separated into three components, V,, V, and V,, which differ by their heavy chain composition ([7, 81 Schwartz