The afferent projections to the olfactory cortical areas from the olfactory bulb and the prepiriform cortex have been studied in the rat, using the autoradiographic method for demonstrating axonal connections. In order to relate closely the results of these experiments to the structure of the olfactory cortical areas, the cytoarchitectonic characteristics of these areas have also been described. The olfactory cortical areas, which receive a direct input from the olfactory bulb, include the anterior olfactory nucleus, the ventral portion of the tenia tects, the olfactory tubercle, the prepiriform cortex, the nucleus of the lateral olfactory tract, the cortical amygdaloid nucleus, and the lateral entorhinal area. All of these areas are composed primarily of pyramidal cells and have three basic layers: a superficial plexiform layer containing the apical dendrites of the pyramidal cells (layer I), a pyramidal cell layer (layer II), and a deeper polymorphic cellular layer (layer III). In each area layer I may be divided into a superficial portion (IA) and a deeper portion (IB). The autoradiographic experiments have shown that all of the olfactory cortical areas receive projections from the prepiriform cortex as well as from the olfactory bulb, and that these two projections have complementary laminar patterns of termination which are the same in every area. Throughout the olfactory cortex the fibers from the olfactory bulb terminate exclusively in layer IA, in relation to the distal segments of the apical dendrite of the pyramidal cells, whereas the fibers from the prepiriform cortex terminate in layer IB, in relation to more proximal segments of the apical dendrites, and also in layer III. The boundary between the two projections within layer I is very sharp, with minimal overlap. In contrast to this precise laminar organization, there is little evidence for a topographical organization within these Projections.