The sagittalis nucleus of the hypothalamus (SGN) is a small nucleus located in the interstitial area between the arcuate and ventromedial nuclei of the hypothalamus in rats. The SGN exhibits male-biased sexual dimorphism and expresses estrogen receptor α and calbindin-D28K. This suggests a contribution of the SGN to sexually differentiated brain function, but its functional role is unknown. In this study, neuronal activation in the SGN during sexual behavior in male rats was examined by c-Fos immunohistochemistry. The number of c-Fos-immunoreactive (c-Fos-ir) cells was elevated with only exposure to chemosensory cues of estrous females and significantly increased after the first mount. The first intromission and ejaculation did not induce further increases in the number of c-Fos-ir cells in the SGN. These findings suggest that the SGN is involved in regulation of the early phase of male sexual behavior, including motivation.