The distribution pattern and the morphology of calbindin D-28k-immunoreactive neurons were studied in the accessory olfactory bulb of the rat using a monoclonal antibody and the avidin-biotin-immunoperoxidase method. Positive neurons were observed in all layers but the vomeronasal nerve layer. Scarce mono-dendritic periglomerular neurons were calbindin D-28k-immunoreactive. Different morphological types of short-axon cells were calbindin D-28k-immunostained, with different degrees of intensity, in the boundary between the internal and external plexiform layer. In addition, deep short-axon cells located in the granule cell layer were calbindin D-28k-immunopositive. By contrast, previous studies described all cells in the rat accessory olfactory bulb as calbindin D-28k-immunonegative. The staining pattern in the rat accessory olfactory bulb showed both similarities and differences with the distribution pattern of the same calcium-binding protein in the main olfactory bulb.