Three species were studied, the rabbit, opossum and rat. Lesions of the main olfactory bulb caused terminal degeneration, assayed by the Fink-Heimer method, to occur in the ipsilateral olfactory tubercle, prepyriform cortex (including its periamygdaloid part), ventrolateral entorhinal area, and in anterior and posterolateral divisions of the cortical amygdaloid nucleus. The various parts of the ipsilateral anterior olfactory nucleus and the rostroventral end of the anterior continuation of the hippocampus (hippocampal rudiment) also received this projection. Lesions of the accessory olfactory bulb, which receives its sensory input from the vomeronasal (Jacobson's) organ, caused terminal degeneration to occur in the medial amygdaloid nucleus and in a posteromedial part of the cortical amygdaloid nucleus. This projection was conveyed by an accessory olfactory tract, which is accompanied in part of its course by a small nucleus, the bed nucleus of the accessory olfactory tract. The accessory olfactory tract is initially a part of the lateral olfactory tract but becomes increasingly indivuated at more posterior levels. It parts company with the lateral olfactory tract at the rostral end of the amygdaloid region, and, in addition to distributing to the medio-cortical amygdaloid region, it enters the stria terminalis to terminate in the bed nucleus of the stria terminalis in a small region bearing cytoarchitectonic resemblance to the medial amygdaloid nucleus. The topographic segregation of the areas of termination of the olfactory and accessory olfactory (vomeronasal) projections is suggestive of a functional dichotomy in the organization of the olfactory system...