Previous studies have provided evidence that pineal-mediated gonadal atrophy involves the luteinizing hormone-releasing hormone (LHRH) system in some rodents. The purpose of the present study was to visualize the LHRH system in the brain of the white-footed mouse by immunocytochemical methods and to assess the influence of blinding or melatonin on the LHRH immunostaining characteristics of the median eminence. Six intact adult female mice and six adult males (two intact, two blinded, and two with subcutaneous melatonin-beeswax implants) treated for 6 wk were sacrificed by perfusion of fixative, and their brains were removed, cut into 60-micron sections and stained for LHRH by the peroxidase-antiperoxidase (PAP) method. Since the tissues were batch-processed in separate wells in a common chamber, semiquantitive comparisons may be made between similar sections from animals subjected to different treatments. The testes were also collected and prepared for microscopic examination. The distribution of LHRH-stained cell bodies and fibers was found to be essentially the same as has been described in other rodents, i.e., predominantly confined to the preoptic area and hypothalamus. The perikarya appear as unipolar and bipolar fusiform cells with "beaded fiber" type processes. In sections that included the median eminence of the blinded and melatonin-treated males, the degree of immunopositive staining appears to be greater than in the median eminence of untreated mice. The histological appearance of the testes from the treated animals revealed a profound inhibition of gametogenesis. These observations support the suggestion that pineal-mediated gonadal atrophy is the result of an inhibition of LHRH release from the median eminence.