The three-dimensional architecture of intracellular membranous structures in dog heart muscle cells was studied by a field emission scanning electron microscope (SEM) using the Osmium-DMSO-Osmium method. Sarcoplasmic reticulum, T-tubule, mitochondria, intercalated disc, surface vesicles and sarcolemma were definitely observed in three dimensions, and the relationship of sarcoplasmic reticulum to other intracellular structures was clearly demonstrated. Especially, the connection between T-tubules and sarcoplasmic reticulum was sometimes observed. On the surface of intercalated disc, small holes of 100 nm in diameter which were lined by a boundary layer were seen among papillary projections. Their functional role remains unsettled.