Total and partial meso-diencephalic transections and lesions of the central gray matter were performed to trace with the Fink--Heimer silver impregnation method the ascending brain stem pathways to the forebrain. Most of these pathways are located near the midline at the level of their entry into the diencephalon. Fibers directed to the hypothalamus either leave the central gray matter and proceed rostralwards in the periventricular tissue layers, or, after traversing the ventral tegmental area, join the medial forebrain bundle. It can be established on the basis of electron microscopical studies that the ascending brain stem pathways project to all hypothalamic nuclei and to the median eminence.