We previously reported that the BST, as the medial preoptic area (mPOA), in female rats has significantly more CRH neurons than in male rats. In the present study, we examined whether low dose testosterone propionate (TP) affected sex difference in CRH neurons in association with changes in the estrous cyclicity. Neonatal female rats were injected sc with 1 μg or 5 μg TP at the age of 5 days. Control male and female rats were injected with sesame oil alone. Daily virginal smears were obtained from the age of 7 weeks and rats were seved to immunocytochemical processing at the age of 9-11 weeks. In control male and female rats, there was a significant sex difference in the number of CRH neurons in the BST and the mPOA, as we reported previously. One μg TP had no effect on the estrous cyclicity and the number of CRH neurons both in the BST and the mPOA. Sixty% of female rats injected with 5 μg TP were persistently estrous. The number of CRH neurons in the BST of these persitent estrous rats was significantly small, but that in the mPOA was not changed, compared to control female rats. We suggest that a low dose testosterone causes loss of sex difference in the number of CRH neurons in the BST and the estrous cyclicity in female rats and that CRH neurons in the BST is more sensitive than the mPOA to steroid milieu during the neonatal period. [Jpn J Physiol 54 Suppl:S221 (2004)]