The molecular mechanism underlying circadian rhythmicity within the suprachiasmatic nuclei (SCN) of the hypothalamus has two light-sensitive components, namely the clock genes Per1 and Per2. Besides, light induces the immediate-early gene c-fos. In adult rats, expression of all three genes is induced by light administered during the subjective night but not subjective day. The aim of the present study was to ascertain when and where within the SCN the photic sensitivity of Per1, Per2 and c-fos develops during early postnatal ontogenesis. The specific aim was to find out when the circadian clock starts to gate photic sensitivity. The effect of a light pulse administered during either the subjective day or the first or second part of the subjective night on gene expression within the rat SCN was determined at postnatal days (P) 1, 3, 5 and 10. Per1, Per2 and c-fos mRNA levels were assessed 30 min, 1 and 2 h after the start of each light pulse by in situ hybridization histochemistry. Expression of Per1 and c-fos was light responsive from P1, and the responses began to be gated by the circadian clock at P3 and P10, respectively. Expression of Per2 was only slightly light responsive at P3, and the response was not fully gated until P5. These data demonstrate that the light sensitivity of the circadian clock develops gradually during postnatal ontogenesis before the circadian clock starts to control the response. The photoinduction of the clock gene Per2 develops later than that of Per1.