The hypothalamic arcuate nucleus (ARC) was electron microscopically studied in diestrous (D) and estrous (E) mice, and mice ovariectomized (OX) 3 weeks before. In the middle region of the ARC (mARC), the number of dense core vesicles (DCVs) appearing in a section of each neuronal perikaryon was examined, and the location of the neurons containing DCVs was mapped. About 10% of the neurons in the mARC contained many (5 or more) DVCs in the perikarya in a section. These neurons mainly localized in the ventromedial portion of the mARC, which shows that the cytoarchitecture of the mouse ARC is heterogeneous. The localization and the frequency of these neurons were not influenced by the variance of ovarian activity. A DCV was composed of an electron-dense core and a membrane halo, and was likely to be formed in the cisternae of the Golgi apparatus. The size of DCVs was estimated at 100 nm as the median diameter and was stable independent of the ovarian activity. However, the formation of DCVs was likely influenced by the ovarian activity, because DCVs in D mice were more intimately associated with the Golgi apparatus than in E or OX mice. In the neuronal perikarya of the ventromedial portion of the mARC, Nissl bodies developed in OX mice, and nucleolus-like bodies developed in E mice. These results suggest that the neurons of the ventromedial portion of the mARC change their function according to the variance of ovarian activity and, therefore, are involved in the estrogen feedback mechanism.