The basal forebrain is critically involved in functions representing the highest levels of integration. Only recently has a relatively clear anatomical picture of this important area begun to emerge. The territory that has generally been referred to as the "substantia innominata" appears to be composed of portions of three recognizable forebrain structures: the ventral striatopallidal system, the extended amygdala and the magnocellular corticopetal system. (1) Rostrally, the striatopallidal system reaches ventrally to the base of the brain. (2) Caudal to the ventral extension of the striatopallidal system elements of the centromedial amygdala and bed nucleus of the stria terminalis are merged so that these two areas together with this subpallidal corridor form a large forebrain unit that might be described as an "extended amygdala". (3) Large cholinergic and non-cholinergic corticopetal neurons form a more or less continuous aggregate that is interwoven with the striatopallidal and extended amygdala systems in basal forebrain. Consideration of morphological and connectional characteristics of basal forebrain suggests that the corticopetal cell groups, together with magnocellular elements of the striatum, serve similar functional roles for the striatopallidal system, the extended amygdala, and the septal-diagonal band complex. Specifically, the output of medium spiny neurons in striatum, extended amygdala, and lateral septum are directed toward somewhat larger sparsely or moderately spiny neurons with radiating dendrites which in turn project to diencephalon and brainstem or provide either local feedback (e.g. in striatum) or distal feedback to cortex. The functional implications of this parallel processing of descending forebrain afferents are discussed.