Ambient light alters the level of the transcriptional regulatory protein c-Fos in the suprachiasmatic nucleus, the site of an endogenous circadian clock in mammals, and in one other retino-recipient area, the intergeniculate leaflet of the lateral geniculate complex. Complementing previous work by ourselves and others on the photic and temporal regulation of c-Fos expression in the suprachiasmatic nucleus, the present studies investigated c-Fos regulation in the rat intergeniculate leaflet, revealing some important differences between the two brain regions. In the intergeniculate leaflet, the levels of c-fos mRNA (by in situ hybridization) and immunoreactive c-Fos protein (by immunohistochemistry) were elevated by light pulses administered either during the subjective day or subjective night. The regulation of immunoreactive Fos-B protein was similar to c-Fos, and 98% of Fos-B-expressing cells were also c-Fos-positive (by double-label immunofluorescence). By combining c-Fos immunofluorescence with stereotaxic injections of the retrograde tract tracer FluoroGold, we found photically-induced c-Fos in 15% of intergeniculate leaflet neurons projecting to the suprachiasmatic nucleus and in 34% of those projecting to the contralateral intergeniculate leaflet. Intergeniculate leaflet cells that express c-Fos after photic stimulation appear to represent a functionally-defined population that does not correspond to anatomically-defined categories based on connectivity or peptidergic phenotype.