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We studied NGFI-A gene expression in response to photic stimulation in the rat suprachiasmatic nucleus (SCN) using in situ hybridization histochemistry. This gene expression spread within the SCN and extended dorsally into the anterior hypothalamus after 30 min-1 h of light exposure at circadian time (CT) CT18. It appeared first in the ventrolateral SCN where the retinohypothalamic tract (RHT) innervates, then it expanded dorsomedially in the SCN and beyond the SCN to the anterior hypothalamus. However, stimulation for 2 h light exposure decreased its expression in the SCN. NGFI-A expression in the somatostatin neurons in the periventricular nucleus increased from 8.7% to 41% with increasing exposure time from 5 to 30 min. NGFI-A mRNA expression in the SCN was suppressed by pretreatment with baclofen, the GABAB receptor agonist. The spread of photic information from the retina to the SCN was visualized at immediate early gene level not only in the SCN but also in the area beyond the SCN. Somatostatin neurons in the periventricular nucleus which project to the external layer of the median eminence and are involved in regulation of growth hormone release showed NGFI-A gene expression corresponding to the duration of photic stimulation. Photic-induced NGFI-A gene expression in the SCN was also shown to be regulated by GABAergic transmission via GABAB receptors. These NGFI-A gene-expressing cells in the SCN may be involved in the circadian entrainment by light and some of those outside the SCN may participate in the regulation of neuroendocrine function.