In the female lamb, early postnatal photoperiod treatments do not alter the timing of puberty as do treatments at later ages. In the male lamb, early photoperiod treatments also fail to influence reproductive development. This prompted the hypothesis that the very young lamb may be unable to transduce changes in daylength into appropriate endocrine cues for puberty. The hypothesis was evaluated by determining the ontogeny of pineal melatonin secretory patterns in both female (n = 4-6) and male (n = 4) lambs under natural photoperiods. Serum melatonin concentrations were determined by a modified RIA in samples collected hourly for 24 or 48 h. Clustering analysis was used to define elevated and nonelevated periods of melatonin secretion. Elevated secretion was evident at night in six of eight lambs by 1 week of age and in all lambs by 3 weeks of age. In nearly all cases, the elevated nighttime levels accurately and consistently reflected the duration of the dark period. The mean amplitude of the elevated nighttime melatonin secretion was low in very young lambs and increased with age. No difference between females and males was observed in either the amplitude or duration of the nocturnal melatonin rise. Our results do not support the hypothesis that the failure of early photoperiod treatments to influence reproductive development is due to an inability of the young lamb to transduce photoperiod patterns into an endocrine signal. Rather, they suggest that some other aspect of reproductive neuroendocrine function is restrictive at these ages.