Recent reports showing that circadian light cues may induce gene-specific patterns of expression in periventricular hypothalamic neurons has extended the functional correlates of light-pulse stimuli, which are conventionally restricted to circadian phase-setting actions within the suprachiasmatic nucleus (ScN). The aims of the present study were, first, to broaden these observations to the protein level using an Egr-1/NGFI-A antibody, and second to investigate the cell-type specificity of induction with respect to associated hypophysiotrophic neurons. In order to co-localize Egr-1 with neuroendocrine peptides, a same-species, double immunofluorescence protocol has been used. The Egr-1 antibody used for immunohistochemistry was first characterized by Western analysis, and we have shown that the C19 antisera detects a single 75 kDa protein species, which has a primarily nuclear subcellular localization. Immunohistochemical analysis has shown that Egr-1 protein is markedly induced by a 1 h nocturnal light pulse both in the body of the suprachiasmatic nucleus and in a dorsal ScN zone, which extends up into the periventricular nucleus (PeN). In contrast, induction of Egr-1 was not observed within the arcuate region of the hypothalamus. Double immunofluorescence histochemistry has shown that Egr-1 is extensively, although not exclusively, co-localized with somatostatin in PeN neurons. The selective induction of the 75 kDa protein product of the egr-1 gene in PeN somatostatinergic neurons, as contrasted with hypophysiotrophic (somatostatin and growth hormone-releasing hormone [GH-RH]) neurons of the arcuate nucleus is indicative of a cell- and stimulus-specific neuroendocrine paradigm, which may be used to address temporal characteristics of somatostatin function that determine pulsatile growth-hormone secretion.