Sixteen pinealectomized and 19 unoperated ewes were exposed to constant light for about 4 weeks before and 4 weeks after lambing. Five ewe lambs born to pinealectomized ewes were implanted s.c. with melatonin sachets and 6 ewe lambs implanted with empty sachets. The 6 ewe lambs born to unoperated dams received empty implants. Ewes and lambs were then returned to the field. Puberty (determined by weekly progesterone analysis) was significantly delayed (P less than 0.05) in 4 of the 5 melatonin-treated ewe lambs; mean pubertal age of ewes with empty implants was 44 weeks of age compared to 45, 63, 72, greater than 72, greater than 72 weeks of age for the melatonin-treated animals. Prolactin levels were decreased in the ewes with empty implants in autumn, coincident with the period of cyclic ovarian activity. No significant decrease in prolactin was observed in autumn in melatonin-treated lambs. Growth was not affected by melatonin treatment. These results indicate a role of the pineal gland and melatonin in the control of puberty in sheep.