It is intended that the proposed work explain the mechanism of spontaneous contraction and relaxation in isolated myocardial fragments which are devoid of functional sarcolemmae. By exposing beating fragments to increasing concentrations of Ca ions, we will determine if there is a threshold (Ca ions) at which contraction begins. If this occurs it will provide support for the regenerative release theory. If not, then this theory will be refuted insofar as this system is concerned. Other experiments will be concerned with actual measurements of calcium flux. We have already shown that sequestration of calcium by the muscle fragments is dependent on ATP, time, temperature and protein concentration. A concentration gradient of several thousand can be generated in phosphate medium. We propose to study the effect of (Ca ions) on sequestration, and then proceed to washout experiments. It is hoped that these experiments will provide an explanation of how spontaneous beating can occur without sarcolemmal function, and perhaps how contractility is controlled in intact muscle.