The three-dimensional structure of the transverse-axial tubular system, sarcoplasmic reticulum (SR), and intercalated disc of the rat left ventricle was examined by high-resolution scanning electron microscopy after removal of the cytoplasmic matrices by the osmium-DMSO-osmium procedure. In the intermyofibrillar space, the transverse tubules (T-tubules) are accompanied by longitudinally oriented axial tubules and together form a transverse-axial system. The junctional SR is usually small but occasionally medium or large in size and couples with the T- or with the axial tubules. On the surface of the junctional SR facing the T- or the axial tubule, tiny junctional processes are seen. One or two sarcotubules, the so-called Z-tubules, frequently run parallel to the T-tubule. The sarcotubules derived from the junctional SR or from the Z-tubule run longitudinally or obliquely and form polygonal meshes around the myofibrils. On the surface of the SR at the H-band level, small fenestrations of 12-40 nm in diameter, and tiny hollows 8-20 nm in diameter are seen. Bulbous swellings of the SR, the corbular SR, are preferentially seen near the Z-band. The large and flat SR, known as the cisternal SR, intercalates among the SR meshes. In the subsarcolemmal space, the sarcotubules form a multilayered network (peripheral SR). The cisternal SR is frequently intercalated in these meshes and closely associated with the inner surface of the sarcolemma. The intercalated disc appears as a prominently undulated membrane demarcating the border between two adjacent heart muscle cells, and occasionally small projections 60-90 nm in diameter and 200-600 nm in length display on its surface.