Our earlier electrophysiological work provided evidence of a direct input to the supraoptic nucleus (SON) from the olfactory bulbs; however, these experiments could not determine if the input originated in the main and/or accessory portions of the olfactory bulb. Here, a connection between the accessory olfactory bulb (AOB) and the SON of the rat was examined using a combination of anatomic techniques. We employed neurophysin immunocytochemistry to delineate the morphological boundaries of the SON and the proximal arborizations of supraoptic dendrites. Accessory olfactory bulb efferents to the SON were studied by injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the AOB. The distribution of retrogradely labeled cells within the AOB was also determined after injection of either rhodamine-labeled latex microspheres (rhodamine beads) or Fluoro-Gold (FG) into the SON. Neurophysin immunocytochemistry revealed that SON dendrites extended beyond the generally accepted boundaries of the nucleus, coursing ventrolaterally along the surface of the periamygdaloid cortex. Anterograde tract tracing with WGA-HRP labeled AOB efferents including a dense plexus of terminals and fibers around the ipsilateral SON along the path of the ventrally projecting dendrites. Injections of retrograde tracers into the SON resulted in rhodamine bead or FG labeling of mitral cells throughout the ipsilateral AOB. Taken together, these anatomic studies suggest a direct projection from the accessory olfactory bulb to the SON of the rat and thus a vomeronasal organ to SON pathway. © 1992.