With the use of rabbit anti-LH-RH serum, a system of delicate nerve fibers and terminals which contain immunoreactive LH-RH can be demonstrated in the median eminence of the rat, corresponding exactly to the tubero-infundibular tract as described earlier with the aid of classical neurophistological methods. The delicate fiber system can be traced to the region of the rectrochiasmatic area and especially to that of the arcuate nucleus but not farther. Nerve cells do not show any immunopositive reaction for LH-RH in the experimental conditions studied. LH-RH positive nerve fibers terminate on or in the immediate vicinity of the capillary loops penetrating the median eminence.