A combination of electrophysiological and anatomical techniques was used to determine the sites of termination of olfactory projections to the thalamus and the distribution of the cells of origin of these projections within the olfactory cortex. Following electrical stimulation of the olfactory bulb, short-latency unit responses were recorded not only in the central segment of the mediodorsal thalamic nucleus but also in the ventral and anterior parts of the submedial thalamic nucleus. Responses were not obtained in the ventral or lateral parts of the mediodorsal nucleus, in the dorsal part of the submedial nucleus, or in the intralaminar nuclei between the mediodorsal and submedial nuclei. The cells of origin of the projection were identified by making injections of horseradish peroxidase conjugated to wheat germ agglutinin (HRP WGA) into the thalamus and examining the olfactory cortex for retrogradely labeled cells. Following injections into the mediodorsal nucleus, labeled cells were found in the polymorphic cell zone deep to the olfactory tubercle, in the ventral endopiriform nucleus deep to the piriform cortex, and in an equivalent position deep to the periamygdaloid and lateral entorhinal cortices. After injections into the submedial nucleus, a smaller number of labeled cells were found in similar locations, except that they were restricted to the rostral olfactory cortical areas and were not found deep to the lateral part of the piriform cortex. Retrogradely labeled cells and anterogradely labeled axons were also found in the lateral orbital and ventral agranular insular areas of the prefrontal cortex with injections into the mediodorsal nucleus, and in the ventrolateral orbital area with injections into the submedial nucleus. Anterograde tracing experiments, using the autoradiographic method, have confirmed these results. Injections of 3H-leucine deep to the junction between the anterior piriform cortex and the olfactory tubercle label axons in both the central segment of the mediodorsal nucleus and the ventral part of the submedial nucleus, while injections deep to the posterior piriform cortex label axons in the mediodorsal nucleus only. Within the mediodorsal nucleus, the projection also appears to be organized so that fibers which arise more rostrally terminate ventrolaterally in the central segment, while fibers which arise more caudally terminate more dorsomedially. These results indicate that there is a substantial and possibly dual thalamocortical mechanism available for processing of olfactory stimuli.