The research proposed will be concerned with various aspects of muscle contractility, especially in cardiac muscle. These are normally rhythmic and the basis of the rhythmicity is under investigation using mammalian cardiac cells in tissue culture. Studies will be continued of the membrane characteristics in relation to high energy phosphate compounds within the cells. The mechanics of the cardiac muscle is dominated by the tension length curve in which passive tension appears on the rising limb of the developed tension curve. I have recently suggested that this characteristic is due to the large series elastic component. This in turn is ascribed to non uniformity of the stress across the cardiac cell due to the organization of tension transmission from cell to cell. Measurements will be made of series elasticity in heart muscle from rabbits and from the horseshoe crab limulus. Thin strips will be taken and studied for sarcomere length using a fine laser beam to give a diffraction pattern from a small length of the muscle. The diffraction lines will be studied for width and uniformity at rest and inactivity. Studies will also be made of the relaxation characteristics of the contractile machinery. In skeletal muscle the relaxation phase under certain conditions has a slope discontinuity known as Funkische's Nase. Thus will continue to be studied by quick release and restretch in order to determine the true kinetics which govern the break of the cross bridge attachment.