The ventromedial nucleus of the hypothalamus (VMN) of male and female rats was examined ultrastructurally at 100 days of age. Axodendritic shaft and spine synapses were counted in two subdivisions of the nucleus, the dorsomedial part (DM-VMN), which contains only a few sex steroid-concentrating neurons, and the ventrolateral part (VL-VMN), which is abundant in such neurons. In normal males, the numbers of shaft and spine synapses were significantly greater in the VL-VMN than in the DM-VMN. In normal females, however, there was no significant difference in the numbers of shaft and spine synapses between the DM-VMN and the VL-VMN. Moreover, the numbers of shaft and spine synapses in the VL-VMN were significantly greater in normal males than in normal females. Castration of males on day 1 significantly reduced the numbers of shaft and spine synapses in the VL-VMN to the level comparable to those of normal females. In contrast, neonatal treatment of females with 1.25 mg testosterone propionate (TP) on day 5 significantly increased the numbers of shaft and spine synapses to the levels comparable to those of normal males. In the DM-VMN, there were no significant differences in the numbers of shaft and spine synapses among normal and experimental animals. These results suggest that the synaptic organization in the VMN is sexually dimorphic but the occurrence of this structural difference is limited to the VL-VMN which is abundant in sex steroid receptors, and is modified by neonatal sex steroid environment.