The term supramammillary area refers to the location of a group of small, variously described nuclei defined by Nissl stain in the posterior hypothalamic region. It is located dorsal to the medial mammillary nucleus and lateral mammillary nucleus, ventral to the posterior hypothalamic area, and ventromedial to the lateral hypothalamic area ( Pan-2004) . It is not well defined in the human ( Crosby-1962; Saper-2004 ), but is more clearly identifiable in the macaque ( Jones-2005; Bleier-1984 ). It is most clearly defined in the rat ( Swanson-2004 ) and mouse ( Hof-2000 ), where it is also commonly referred to as the 'supramammillary nucleus'. In the rat and mouse it is divided into two parts: the medial part of the supramammillary nucleus and the lateral part of the supramammillary nucleus. Functionally it is part of the behavioral state system ( Swanson-2004 ).