Fifteen minutes of bright, white light exposure at midsubjective night resulted in a marked increase in both the number and stain density of c-fos protein (Fos)-immunoreactive neurons in the suprachiasmatic nuclei (SCN). In all cells, peroxidase reaction product was confined to the nucleus. Most Fos-immunoreactive cells were concentrated in the ventrolateral third of the SCN, although a few immunoreactive cells were also observed diffusely distributed along the dorsal border of the nucleus and into the surrounding hypothalamus. Along the rostro-caudal extent of the SCN, the greatest density of Fos-immunoreactive cells was found at a level approximately 300-400 microns caudal of the rostral pole of the nucleus. The population of Fos-immunoreactive cells in the SCN lies within the terminal fields of both the retinohypothalamic and geniculohypothalamic tracts. In addition, a few Fos-immunoreactive cells were observed in the ventral lateral geniculate nucleus. The results suggest that retinal illumination induces transsynaptic c-fos expression in a select population of SCN neurons.