The aim of this study is to determine whether sex-related differences exist in the biosynthetic activity of the mitral cells within the mitral layer of the AOB. Possible functional changes over the estrus cycle and the potential effects of castration and androgenization are assessed. Biosynthetic activity was measured using silver staining of the argyrophilic proteins associated with the nucleolar organizer regions (Ag-NOR). Assisted by stereological methods, the following parameters were studied: mean number, percentage and mean area of Ag-NOR in estrus and diestrus females, intact males, castrated and androgenizated rats. We detected sex differences in a histochemical marker related to synthetic activity, an estrus cycle effect and changes resulting from the perinatal treatments. We conclude that this structurally dimorphic region is also functionally dimorphic.