Neuron number in the principal nucleus of the bed nucleus of the stria terminalis (BNSTp) is greater in adult male mice than in females. Deletion of the proapoptotic gene,Bax,increases the number of BNSTp cells in adulthood and eliminates the sex difference in cell number. Here, we map the ontogeny of sex differences in nuclear volume and cell number in the BNSTp of neonatal mice, and evaluate the role of cell death in the development of these differences. We find that BNSTp volume and cell number do not differ between male and female wild-type mice on postnatal days P3, P5, or P7. Sex differences emerge after the first postnatal week and both measures are significantly greater in males than in females on P9 and P11. Cell death, assessed by TUNEL staining, was observed in the BNSTp of both sexes from P1-P8. Females had more TUNEL-positive cells than males from approximately P3-P6, with the maximum number of dying cells observed on P5/P6. To test whether theBaxgene is required for sexually dimorphic cell death in the BNSTp, TUNEL cells were counted on P6 inBax/ mice and theirBax+/+ siblings.Baxgene deletion nearly abolished TUNEL-positive cells in the BNSTp of both sexes. Together, these findings support the interpretation that the sex difference in BNSTp cell number seen in adulthood is due toBax-dependent, sexually dimorphic cell death during the first week of life.