The ontogenesis of Retzius-Cajal cells, a unique feature of developing cortical layer I in a variety of mammalian species, was examined with the electron microscope in coronal or tangential sections of the visual cortex of rats whose ages were closely spaced in time between day 17 of gestation and adulthood. At 17 days of gestation, Retzius-Cajal cells already display a characteristic appearance and some of the cytoplasmic organelles by which they are identified in the perinatal period. At birth they are recognized by their large size, horizontally oriented long processes, dark cytoplasmic ground substance and abundance of tightly packed organelles. One feature which is most typical of these cells at this, and later stages of development, is the presence in the cytoplasm of numerous wide cisterns of granular endoplasmic reticulum filled with electron-opaque material. Synapses are rarely seen on the perikarya and processes during the first week of postnatal life but become more frequent later in development. A pattern of modifications becomes noticeable in the morphology of these cells during the first postnatal week with the appearance of growth cone-like differentiations and new processes of varying sizes. Furthermore, their cytoplasm slowly acquires a lighter appearance, and the thickness of the characteristically long processes diminishes. The frequency of Retzius-Cajal cells decreases with age and at the end of the third postnatal week only very few can be recognized with certainty. Careful examination of a large series of sections during subsequent days revealed that the morphological characteristics of Retzius-Cajal cells continue to change until these cells can on longer be distinguished from classical layer I nonpyramidal neurons.