Recent work has suggested that immediate early genes are involved in the phase-shifting response of the circadian pacemaker to light. In mammals, the endogenous pacemaker is located in the suprachiasmatic nucleus (SCN). We report here a systematic examination of the expression of the immediate early genes fos, NGFI-A, and NGFI-B in the rat SCN, in basal and light-stimulated conditions across the circadian cycle. The photic induction of all three genes examined was found to be gated by the circadian system with maximal induction observed during the mid to late subjective night. The photic-induced expression of NGFI-B in the SCN was considerably less than that of NGFI-A, despite previous observations suggesting greater induction of NGFI-B by membrane depolarization. The levels of immediate early gene expression induced by light do not directly correspond to the magnitude of previously reported light-induced phase-shifts of the activity rhythm. The results also suggest that fos may undergo a circadian rhythm of expression in the rat SCN.