The neurons in the hypothalamic arcuate nucleus (ARC) of 20-day-old female mice were examined by electron microscopy. The administration of estradiol-17 beta during 20 days from the day of birth resulted in the increase of dense-core vesicles (DCVs) in some neuronal perikarya of the middle region of the ARC (middle ARC), while, in the posterior region of the ARC (posterior ARC), the increase of DCVs was less marked. Topographical studies revealed that the neuronal perikarya containing few DCVs were assembled in the ventromedial area of the middle ARC in both control mice and mice given estrogen. Those containing 5 or more DCVs were found predominantly in the ventromedial area of the middle ARC in mice given estrogen. In the posterior ARC, however, the neuronal perikarya containing DCVs seemed to be scattered. The distribution patterns of the diameter of DCVs were significantly different between control mice and mice given estrogen. The administration of estrogen seem to bring about the enlargement in the size of DCVs. These results indicate that the neurons responding to estrogen are present in the ventromedial area of the middle ARC, and suggest that this area is a significant focus in estrogen feedback mechanisms.