The structural alterations produced by transection in striated fibers of cremaster muscle are studied with light and electron microscopes. Diffusion of procion yellow and procion black, which are extracellular space markers, is observed after incubating the transected fibers in the presence or in the absence of Ca++ for 1 h. Cytoplasm at the injured end is always separated from the extracellular space by the sarcolemmal membrane. No outflow of cytoplasmic structures can be seen with light or electron microscopes. The zone of muscle fiber adjacent to the transection is generally dilated. It is occupied by a meshwork of 7-nm-thick filaments. No normal myofibrils are present. The diffusion of procion markers comes to an end within 1 h after the lesion if the fiber is incubated in normal Krebs’ solution. In this period of time they penetrate 250–350 µm inside the fibers. When the transected fibers are incubated in a Ca++ -free solution, procion dyes penetrate about 1 mm. This distance increases in the following 3 h. Diaphragm muscle fibers are used as controls. In these fibers the free penetration of the markers by the injured end is not impeded by Ca++ . These results are in accordance with the calcium-dependent recuperation of the membrane potential 1 h after the transection in cremaster muscle fibers, and with the absence of this recuperation in the diaphragm. It is proposed that the closure of the sarcolemmal membrane creates the diffusion barrier blocking the penetration of markers and ions, allowing the recuperation of membrane poten