The subcommissural organ of the rat has been studied by a combination of light, scanning and transmission electron microscopy. Special attention has been paid to the appearance of the ventricular surface of the organ. The ependyma-derived cells on the surface of the organ are extremely long with a width which diminishes steadily from the base, where the nucleus is situated, to the ventricular surface. This results in a fan-shaped arrangement of cells, the cytoplasm of each cell reaching the surface of the ventricular system. The apical surface of the cell has small ciliated extensions 1-1.5 micrometers in diameter protruding into the ventricular lumen. With conspicuous rough endoplasmic reticulum and Golgi apparatus, and microtubules in profusion, the subcommissural organ cells possess the hallmarks of active secreting cells. The surface of the cells shows pinocytic vesicles-suggesting exchange of materials with the cerebrospinal fluid.