In the present study the morphogenesis of the ventricular ridges, i.e. the site of origin for neurons in the basal ganglia and various related basal forebrain structures, has been studied in the Chinese hamster with the aid of three-dimensional and graphical reconstructions. The first ridge appears at developmental stage 14 (E 12 1/2). It originates at the level of the torus hemisphaericus, thereby obscuring the basal part of the telodiencephalic boundary. Later on this ridge passes into the medial ventricular ridge. Subsequently, the lateral ventricular ridge arises at stage 16 (E 13 1/2). Initially, both ridges are completely separated by the sulcus subpallii intermedius. During further development, however, this limiting groove fades away, a process starting caudally and gradually proceeding in the rostral direction. Eventually, this process results in the formation of one single ventricular eminence at the second fetal stage (E 18). In the adult stage the ventricular eminence curves around the cerebral stem area. Thickening of the telencephalic walls and local coarctations have considerably reduced the lumen of the lateral ventricle. The preoptic region in the adult must be considered as a derivative of the diencephalic part of the medial ventricular ridge.