It is well established that photic cues are used by the suprachiasmatic nucleus (SCN) to entrain circadian rhythms to the light/dark cycle, but the role of photic stimuli in the regulation of ultradian neuroendocrine rhythms is ill defined. In relation to the rhythms of GH secretion, recent studies have shown that nocturnal photic stimulation induces gene expression not only in the SCN but also in periventricular (PeN) somatostatin (SRIF) neurons. We have, therefore, investigated the effect of nocturnal photic stimulation on spontaneous and induced GH secretion in conscious rats. Nocturnal photic stimulation (lights on at 2400 h for 1 h) suppressed spontaneous GH secretion in male and female rats and reduced the GH response to SRIF withdrawal and iv injection of GH-releasing factor. A similar trough in GH secretion was also observed during the first hour of the normal light phase (0600 h). Using immunohistochemical analysis, we have also shown that expression of the transcription factor, Egr-1, is induced at the commencement of the light phase in the SCN, PeN, and medial preoptic nucleus. This effect is abolished by maintaining rats in the dark during this period. These data, together with our previous demonstration that 50% of SRIF-positive neurons in the PeN coexpress Egr-1 after photic stimulation, suggest that activation of SRIF neurons in the PeN may entrain the episodes of GH secretion to the dark/light interface. However, the absence of synchrony in GH pulses between animals by the second half of the light period suggests that this entrainment is transient.