When various cations, including Ca2+, are in the fixative, both sarcoplasmic reticulum (SR) of whole skeletal muscle and isolated SR vesicles collapse to form pentalaminate "compound membranes" that result from the apparent fusion of the lumenal lamellae of the membranous envelope of the SR. The process may be reversed by subsequently soaking the tissue in 1 M NaCl. An identical morphological phenomenon is observed in unfixed quickly frozen isolated frog skeletal muscle fibers, the cation in that case coming from endogenous sources. The hypothesis is advanced that the collapse is an in vivo process mediated by the sequestration of Ca2+ after contraction. The resulting obliteration of the SR lumen would have the effect of displacing the SR contents into the junctional SR, as well as electrically isolating the free SR from the junctional SR during relaxation. As a consequence, resistive coupling between the plasmalemma and the junctional SR becomes a plausible mechanism for the translation of the action potential into Ca2+ release, since the bulk of the SR membrane capacitance would now remain separated from the plasmalemma during relaxation.