In the spring-born lamb, puberty, the onset of repetitive ovarian cycles, occurs between 25 and 35 weeks of age during the decreasing day length of autumn. This study was conducted to determine the photoperiod conditions required for puberty. Several artificial photoperiod treatments were used, beginning from birth or soon after. Continuous exposure to long days or to short days delayed the first normal luteal cycle beyond 1 yr of age. Similarly, a single block of 4 or 10 weeks of long days at early ages, from either 3-7 or 3-13 weeks of age (afterwards under short days), produced only a few consecutive cycles within the first year after birth. However, exposure to blocks of long days at later ages resulted in the onset of cycles during the normal pubertal period. Lambs that experienced a 10-week block of long days (12-22 weeks of age; otherwise under short days) began reproductive cycles at 34 weeks of age. Reduction of the single block of long days to 5 weeks (17-22 weeks of age) or 1 week (week 22 of age) also resulted in the onset of normal luteal cycles within the normal age range for puberty. The results indicate that a sequence of long days followed by short days is required to initiate and sustain ovulatory cycles at the normal age in the lamb. Moreover, the age when long days are experienced is important, with later exposure to long days being more effective in producing consecutive cycles than early exposure to long days. These findings raise the possibility that the spring-born lamb uses a portion of the long days of summer to time the onset of puberty during the short day lengths of autumn.