Tritiated thymidine autoradiography was used to study the origin and distribution of neurons in the primary olfactory cortex of the rat. The principal interest was devoted to animals injected at embryonic day 12 (E12) and sacrificed at different pre- and postnatal ages. The first generated neurons appearing at E12 were studied from E15 to P63. Animals sacrificed at E15 show a group of heavily labeled cells occupying a large area of the ventro lateral region of the telencephalic vesicle. At E16 this group differentiates into the principal cells of the accessory olfactory bulb and cells of the prospective primary olfactory cortex (POC). At E18-E20 the ventral tip of the cortical plate apparently divides this group into a superficial part corresponding to layer I and a deep part, corresponding to cells located in the adult in layer III. Labeled cells in layer I were found flanking the lateral olfactory tract (TOL), but rarely in the adult suggesting that they disappear or transform postnatally. Golgi observations were carried out from E15 to postnatal day 8. The morphology of different cells were studied. Layer I contains polymorphic cells resembling Cajal-Retzius cells. Among other cell types, layer II includes kinds of pyramidal cells lacking basal dendrites known as semilunar cells and intrinsic neurons. Layer III contains pyramidal cells having more than one apical dendrite ascending to the surface.