Dense cluster of cells in the lamina terminalis that spreads along the anterior border of the third ventricle from the tip of the preoptic recess of the third ventricle to a triangular area between the descending columns of the fornix. In other words, it is located along the midline of the brain at the anterior end of the hypothalamus bordering the third ventricle. It merges with the organum vasculosum and also receives input from the subfornical organ. The median preoptic nucleus is involved with blood composition and volume in response to hormones and behaviors and regulates body temperature.