The three-component model for muscle proposed by A. V. Hill (1) has been useful in describing the contractile activity of cardiac muscle in vitro (2) and in vivo (3). The contractile element (CE) of this model is assumed to be freely extensible at rest, but with activation it shortens according to a characteristic inverse relation between the velocity of shortening and the load (4). The CE is in series with an elastic element (SE) so that during isometric contraction, the activated CE shortens and stretches the SE, the rate of tension development (dP/dt) being determined by the CE velocity and the stress strain relation of the SE (1). Thus the external manifestations of CE activity depend to a large extent on the properties of the SE. Although the stiffness of the SE is unaffected by inotropic interventions, or the course of active state (5,6), it does become somewhat stiffer following damage from segmental compression (7). With the recent application of cardiac muscle mechanics to the study of pathologic states such as hyperthyroidism, cardiac hyperthophy and failure (8-10), a quantitative knowledge of the SE compliance in these conditions is required if CE velocity and work are to be evaluated. Accordiwly the present study was under taken to measure the series elasticity of papillary muscles from cats with hyperthyroidism, cats with cardiac hypertrophy, and those with cardiac hypertrophy and heart failure. Methods. Right ventricular papillary muscles from three group of cats (1.5-2.5 kg) were used. Hyperthyoidism was induced in six cats by the intraperitoneal injection of 1-thyroxine (1 mg per kg per day) for 10-14 days (10). Serum protein bound iodine (PBI) and cholesterol determinations were made at the time of sacrifice.