The pattern of prolactin (PRL) secretion following mating consists of two daily surges, one nocturnal and the other diurnal. Under a photoperiod of 14L: 10D, the most commonly used lighting regime, the nocturnal PRL surge occurs at about 7 h after the onset of the dark phase. The present study investigated the influence of various photo-periods, constant light and dark and suprachiasmatic nucleus lesion (SCN) on the initiation and expression of the nightly PRL surges. Females were divided into five groups and each group was exposed to a different photoperiod. The five experimental photoperiods were: 22L:2D, 19L:5D, 14L: 10D, 8L:16D and 3L:21D. Following at least 8 weeks of exposure to a photoperiod the females were mated. Multiple daily blood samples for PRL determination (RIA) were obtained via chronic intra-atrial cannulae. In females exposed to a decreased period of darkness the nocturnal PRL surge was advanced with respect to the onset of the dark phase. Conversely, an increase in the length of the dark period to over 10 h/day caused a delay in the appearance of the PRL surge. When the timing of the PRL surge is calculated with respect to the midpoint of the dark or light cycle, it occurs at about the same time under all but the most extreme photoperiod (3L:21D). Prior to the experiments using constant dark or constant light the animals were habituated to the lighting conditions for 4 months. In constant darkness the nocturnal PRL surge was synchronized among all animals and peaked in the early morning; its magnitude and duration were greater than under both constant light and alternating light schedules. In constant light, irregular elevations of PRL were seen throughout the day. No elevation of PRL was found in SCN-lesioned animals following cervical stimulation. The present study demonstrates that the nocturnal PRL surges during pregnancy and pseudopregnancy are entrained to photoperiod, can be shifted within limits under various photoperiods and cannot be initiated following lesions in the SCN. On the basis of this and other studies, it is proposed that the nocturnal PRL surge resembles other endogenous circadian rhythms in the rat, but requires the external stimulation of mating for its occurrence.