Sections of the hypothalami and pituitary glands of normal (Sprague-Dawley) and homozygous diabetes insipidus (Brattleboro) rats were stained with antiserum to a human pituitary glycopeptide (HPGP) by using the immunohistochemical peroxidase-antiperoxidase method at the light microscopic level. Our results show in normal rats that immunoreactive HPGP was localized in the perikarya of the magnocellular neurons of the hypothalamus, in the posterior pituitary, and in the nerve fibers distributed in the median eminence (ME) and in the areas between the supraoptic nuclei (SON), paraventricular nuclei (PVN), and median eminence and also in the suprachiasmatic nuclei (SCN), a part of the parvocellular system. In the Brattleboro rats, however, no staining was found either in the hypothalami or pituitary glands. The present data strongly support our previous hypothesis that HPGP, a 39 residue glycopeptide isolated from human neurohypophysis, may be part of the precursor of arginine-vasopressin and its neurophysin II (Pro-NP-AVP).