In the present study, we investigate the development of sex differences in the bed nucleus of the accessory olfactory tract (BAOT), a sexually dimorphic structure in which masculinization of male rats depends on the level of postnatal estradiol. Male and female Wistar rats were sacrificed postnatally when they were 1, 3, 7, 15, 45 and 60 days old, and stereological methods were used to estimate the volume and number of neurons in the BAOT of the subjects. With respect to volume, sex differences were established from P15. In relation to the number of neurons, the males acquire their adult morphological parameter by P15, while females show a decrease (P3), increase (from P3 to P15) and later decrease (from P15 to P45). Sex differences from P3 to P7 might be related to greater cell death in the BAOT of the females. These results support the hypothesis that both male and female rats reach their adult morphological pattern within this sexually dimorphic nucleus in an active manner.