Fiber degeneration in the rat following lesions of the lateral hypothalamic area was studied by the method of Nauta and Gygax ('54). Ascending degeneration in the medial forebrain bundle enters the preoptic area and the medial and fimbrial nuclei of the septum. Less dense degeneration is found in the lateral septal nucleus and a few fibers bypass the septum and reach the cingulate cortex. Descending degeneration in the medial forebrain bundle enters the ventral tegmental area of Tsai giving off a few fibers to the mammillary bodies on the way. In the midbrain fibers turn dorsomedially to enter the raphe nuclei and the central gray and dorsolaterally to enter the nucleus mesencephalicus profundus pars lateralis. A small group of fibers leaves the anterior part of the lateral hypothalamus in a dorsomedial direction and passes through the inferior thalamic peduncle and stria medullaris to enter the lateral habenular nucleus. Another bundle leaves the posterior part of lateral hypothalamus in a dorsomedial direction and passes through the posterior midline thalamus to enter the central gray. No degeneration was found in the ventromedial hypothalamus in rats with lateral hypothalamic lesions. Our ability to stain fine degenerating fibers in this region was demonstrated in rats with medial hypothalamic lesions.