By means of immunohistochemistry using anti-gamma-aminobutyric acid (GABA) antibodies, characteristic neurons of pyriform, bipolar or pleomorphic shapes, regarded as the Cajal-Retzius (CR) neurons, were clearly demonstrated in layer I of the rat cerebral cortex at the various experimental periods. On embryonic day 15, ovoid neurons only in the marginal zone indicated immunoreactivity for GABA. They gradually extended thick processes often in parallel with the pial surface and formed a dense GABA fiber network in immature layer I during the early postnatal periods. Some GABA neurons seemed to migrate into the underlying layers to settle as nonpyramidal cells. With the expansion of brain volume, GABA neurons relatively diminished and decreased in number. Nevertheless, a small number of GABA neurons did exist as essential CR neurons in layer I even in the adult rats aged 9 months.