In female sheep, photoperiod regulates the timing of the transition to adulthood. We tested the hypothesis that photoperiod very early in development influences the timing of the pubertal LH rise that initiates sexual maturation. The first experiment was designed to determine the influence of day length information perceived before birth by varying prenatal photoperiod experience. Two groups that experienced either increasing or constant long days prenatally, and then a gradually decreasing photoperiod postnatally, reached puberty at the same age (Prenatal Increase, 20.4 +/- 0.5 wk vs. Prenatal Long Days [LD], 19.4 +/- 0.8 wk). Puberty in these groups was much earlier than in two control groups exposed to the same photoperiods, but beginning at birth, for 13 wk (Postnatal Increase, 29.6 +/- 1.0 wk; Postnatal LD, 26.2 +/- 1.3 wk). In the second experiment, the role of prenatal photoperiod in timing sexual maturity was also examined through the use of treatments with greater contrast. Lambs were exposed prenatally to either decreasing or increasing day lengths. Beginning at birth, both groups were exposed to a decreasing photoperiod. Although only half of the lambs in each group exhibited the pubertal LH rise, those that attained puberty did so at the same age (Prenatal Decrease, 14.8 +/- 1.0 wk vs. Prenatal Increase, 14.8 +/- 0.3 wk). We therefore conclude that day length cues experienced postnatally predominantly time sexual maturation in the female lamb.