The term periventricular zone of the hypothalamus refers to one of three zones of the hypothalamus when it is subdivided by planes parallel to the midline. The others are the medial zone of the hypothalamus and the lateral zone of the hypothalamus. The periventricular zone is the closest to the midline and includes the median preoptic nucleus, the preoptic periventricular nucleus, the anteroventral periventricular nucleus, the organum vasculosum, the suprachiasmatic nucleus, the intermediate periventricular nucleus, the arcuate nucleus of the hypothalamus, the neurohypophysis, the posterior periventricular nucleus and the posterior hypothalamic area. Some authors include the subfornical organ in this zone ( Crosby-1962, Saper-1990 ). See also the functionally defined hypothalamic periventricular region.