The structure of the membranes limiting the sarcoplasmic reticulum (SR) and transverse (T) tubules in frog and fish muscle fibers has been studied by freeze fracture. Emphasis is placed on the structure of the membranes at the triad, where thin sections have previously shown that rows of regularly disposed "feet" join SR and T exposed fracture faces allows the following conclusions. 1) The SR membrane is continuous and identical in appearance along the whole sarcomere. Thus the SR is a single uninterrupted compartment and it is likely that the major function of the reticulum, calcium accumulation is performed by the membrane limiting the lateral sacs of the triad, as well as the longitudinal tubules. 2) At the level of the junction with the T tublue, the SR presents a strikingly different number, size and arrangement of particles and pits. This distinct portion of the SR membrane extends father than the area covered by the junctional "feet" and no correlation can be found between the disposition of particles within the membrane and that of the feet on the membrane surface. 3) The T system membrane presents few prominent particles on its junctional face, but these are far less numerous than the feet. 4) Thus, no visible preformed channels exist between SR and T system lumina and it is suggested that direct electrical coupling between the two membranes during excitation is unlikely.