This review is mainly concerned with the presentation of our experimental results and ideas concerning the relaxing factor in the contraction-relaxation cycle of skeletal muscle. The microsomal (or vesicular) relaxing factor, probably identical with the endoplasmic reticulum, is able to bind strongly Ca in the presence of ATP. Since Ca is shown to be essential for the contraction of actomyosin system, this Ca binding seems to represent the mechanism of the relaxing factor. In view of these findings, a possible mechanism of the link between excitation and contraction is postulated.