Cell proliferation in the accessory olfactory bulb of the adult rat was analysed after systemic injection of 5-bromo-2'-deoxyuridine, detected immunocytochemically at different survival times and compared with proliferating cell nuclear antigen immunostaining. As previously described in the main olfactory bulb, local cell proliferation was absent or very limited. By contrast, starting from 15 days after bromodeoxyuridine administration, many immunoreactive nuclei were present in the granular layer, and to a lesser extent, in other layers of the accessory olfactory bulb. This suggests that the newly-generated cells are migrating elements of the rostral migratory stream which are known to reach the olfactory bulb in 15 days. By immunocytochemical detection of the polysialylated isoform of the neural cell adhesion molecule, a weakly-adhesive cell-surface molecule expressed by newly-generated/migrating cells of the rostral migratory stream, we found a high number of immunoreactive cells in the different layers of the accessory olfactory bulb. Most of these cells were observed in the granular layer and showed the morphology of migrating neuroblasts. Some immunoreactive cells displaying neuronal morphology were also detected in the external plexiform and glomerular layers. Double labelling experiments demonstrated that these cells are newly-generated cells. These results demonstrate the occurrence of newly-added cells in the accessory olfactory bulb of the adult rat, which likely correspond to the neuronal precursors originating from the rostral migratory stream. This could be relevant since the accessory olfactory bulb of rodents plays an important role in the hard wiring of a simple olfactory memory system for sexual pheromones.