Circulating prolactin concentrations were monitored during the early postnatal period in sheep to evaluate their response to photoperiod. In the first experiment, male and female lambs were exposed from 1 week of age, with their mothers, to either long days (16 h light: 8 h darkness; n = 15) or short days (8 h light: 16 h darkness; n = 16) to test whether they could discriminate different day lengths. In both sexes, serum prolactin concentrations were higher on long than on short days during the first 7 weeks after birth. In the second experiment, female lambs (n = 21) were raised on long days from 2 weeks of age. The superior cervical ganglia were removed bilaterally at 4 weeks of age from 14 lambs to lesion the sympathetic innervation to the pineal gland, and thus ablate the nocturnal increase in pineal melatonin secretion. After surgery, serum prolactin concentrations on long days were significantly lower in ganglionectomized lambs than in the intact controls. In the third experiment, the amplitude of the night-time melatonin rise was artificially increased in female lambs (n = 8) between 2 and 7 weeks of age to adult levels. Unrestrained lambs were infused during the 8-h dark phase of each day with melatonin by means of a self-contained, computerized syringe-pump. Concentrations of circulating prolactin did not differ from those in uninfused lambs (n = 8) with lower endogenous nocturnal melatonin. These results reveal that the sheep can discriminate photoperiod cues during the early postnatal period, and suggest that the low-amplitude melatonin rhythm in the neonatal lamb is sufficient to mediate this response.