This work investigates the possible existence of sex differences in the number of neurons in the medial posterior region (BNSTMp) and the lateral division (BNSTL) of the bed nucleus of the stria terminalis in the rat. These two zones of the bed nucleus of the stria terminalis belong, respectively, to the vomeronasal system (VNS), and to the main olfactory system (MOS). In the BNSTMp, males showed a greater number of neurons than females. Early postnatal (Day 1 after birth) orchidectomy in males, and androgenization in females, eliminated and reversed these differences. In the BNSTL, sexual dimorphism was restricted to its anterior region (BNSTLa). Females showed there a greater number of neurons than males. Male orchidectomy on Day 1 after birth increased the number of neurons, while female androgenization produced the opposite effect. The results obtained in this study support the hypothesis that the VNS is sexodimorphic, and suggest that sex differences exist in MOS, and that these differences are controlled by gonadal steroids during the perinatal period.