The term interpeduncular nucleus refers to a group of cells located at the midline of the ventral midbrain tegmentum ( Carpenter-1983 ). More homogeneous in the primate than in the rodent, it is variably subdivided by different authors in the different species. In the human it is divided into four subnuclei: rostral subnucleus of the interpeduncular nucleus, apical subnucleus of the interpeduncular nucleus, dorsal part of the lateral subnucleus of the interpeduncular nucleus, and caudal subnucleus of the interpeduncular nucleus ( Paxinos-2012 ), The macaque has three subnuclei: rostral and caudal subnuclei as in the human and a lateral subnucleus of the interpeduncular nucleus ( Paxinos-2009a ). In the rat ( Swanson-2004; Paxinos-2009b ) and the mouse ( Franklin-2008 ) all authors identify five subnuclei: rostral subnucleus of the interpeduncular nucleus, apical subnucleus of the interpeduncular nucleus, lateral subnucleus of the interpeduncular nucleus, dorsal part of the lateral subnucleus of the interpeduncular nucleus, and intermediate subnucleus of the interpeduncular nucleus; they differ with regard to the presence or absence of three other subnuclei. Functionally the interpeduncular nucleus belongs to the behavioral state system ( Swanson-2004 ).