The distribution pattern of cholinesterase activity in the basal forebrain region was examined in five human brains without any history of neurologic or psychiatric disorders. Complete sets of serial sections allowed a three-dimensional reconstruction of this region. The intensity grading and the distribution pattern of the specific and non-specific cholinesterase activity was depicted diagramatically. The distribution pattern of cholinesterase activity in the supracommissural striatum demonstrated the well-known striosomal configuration, particularly in the head of the caudate nucleus. Within this nucleus caudatus the striosomes appeared connected with a subventricular zone of low acetylcholinesterase-activity. Bands of very high activity could be demonstrated from the dorsolateral and ventral areas of the caudate nucleus to the lateral border of the putamen and the commissural and subcommissural division of the ventral striatum. The distribution pattern of cholinesterase activity in the subcommissural region showed very close correlation to the cytomorphological subdivisions of the striatum as defined by Brockhaus (1942). In addition to his topographic description it was possible to define the tuberculum olfactorium and several subdivisions of the interstitial nucleus of the stria terminalis. The inhibition of non-specific esterase activity by ISO-OMPA in the globus pallidus allowed distinction between striatal and pallidal components. Three-dimensional reconstructions of the terminal islands revealed several types, which were named according to their topography as insulae substriatales, -subventriculares, -olfactoriae, -magnae, and -interstitiales. Characteristically, the core of these islands consisted of clusters of tightly packed, extremely high acetylcholinesterase-positive cells. Cholinesterase activity of the surrounding rim region ranged from negative to strongly positive depending on the position and type of the island. The findings suggest that the islands represent derivatives of the fundus striati region as defined by Brockhaus and are connected to the dorsal striatum by means of cellular bridges.