The role of Ca ions in the contractility of the myocardium and skeletal muscle of warm-blooded animals is examined in the light of the published experimental findings. It is shown that in the period of relaxation the concentration of Ca in the sarcoplasmic reticulum (s.r.) of the cardiomyocytes must be about 10-4 m and the corresponding Ca concentration gradient "s.r.-myoplasm" equal to 103 is the maximum possible in the work of the Ca pump (high Ca concentration gradients are the products of individual gradients); in the period of contraction the "s.r.- myoplasm" Ca gradient falls to the level 1-101. It was found that the power of the Ca pump of the s.r. membranes is approximately 102 times higher than that of the sarcolemma membrane. It has been established by virtue of the spatial-morphological features of the structure of the muscle cells that of all the external calcium bound to the sarcolemma only that part of it present in the T-system can take part in the contractile act. On the basis of the structural features of the T-system and the ratio of the volumes and areas of the T-system and s.r. the concept of the "coefficient of utilization in the contractile act of external Ca" is introduced and the differences in the stability of the cardiomyocytes and myocytes of the skeletal muscles to fall in the Ca content in the external medium are explained. © 1987.