The effects of postnatal male castration on day 1 (D1) after birth and female androgenization on accessory olfactory bulb (AOB) light and dark granule cell populations were studied. Control males showed a greater number of both light and dark AOB granule cells than females. Postnatal treatment reversed these differences in the light granule cells. Female androgenization on D1 does not affect the number of AOB dark granule cells. However, male orchidectomy also on D1 significantly decreases the number of dark granule cells. Androgens injected into female rats are able to increase the number of AOB dark granule cells if they are administered on D14. This suggests a different critical period for the early effects of androgens on the dark granule cells with respect to light granule cells.