The sexually dimorphic nucleus (SDN) is an ovoid, densely packed cluster of large cells located in the medial preoptic area (POA) of the hypothalamus which is believed to be related to sexual behavior in animals. Thus far, for all species of mammals investigated, the SDN has been repeatedly found to be considerably larger in males than in females. In humans, the volume of the SDN has been found to be 2.2 times as large in males as in females and to contain 2.1 times as many cells. The human SDN is elongated in females and more spherical in males. No sex differences have been observed in the human SDN in either cell density or mean diameter of the cell nuclei. The volume and cell number of the human SDN considerably decreases with age, although the decrease in cell number is both sex and age-specific. In males, a substantial decrease in the cell number of the human SDN was observed between the age of 50–60 years. Cell death was more common in females than males, especially among those older than 70 years of age. The SDN cell number in females can drop to 10-15% of that found in early childhood.

SDN and its homologues exist widely in human, mammal, and some other animal brains, including:

• the third interstitial nucleus of the anterior hypothalamus (INAH3) in humans;
• ovine sexually dimorphic nucleus (oSDN) in the medial preoptic area/anterior hypothalamus (MPOA/AH) in sheep;
• sexually dimorphic nucleus in the preoptic area (SDN-POA) in rats;
• anterior hypothalamic nucleus (AHdc) in macaques;
• specific area in medial preoptic nucleus (POM) in quails; etc.