Day-night variations of gastrin releasing peptide (GRP) in the neuronal somal area of the SCN and the nearby region on the anterior hypothalamic area (AHA) where GRP fibers project, were examined by semiquantitative immunocytochemistry, and compared with those of vasoactive intestinal peptide (VIP). Both in the SCN and AHA, GRP immunoreactivity was higher during the day than at night, although VIP immunoreactivity was higher at night than during the day. These observations suggest that, although they are produced in the same ventrolateral subdivision of the SCN and, in part, coexist in the same neurons [13], the day-night changes of GRP and VIP immunoreactivity are in the opposite direction to each other.