The concentrations of LH and FSH were measured by radioimmunoassay in sera from immature male and female rats of various ages. Fairly high levels of FSH were found in both sexes at birth but lh was not detected. FSH peaks appeared in the male at 13 and 19 days of age and in the female at 13 and 17-19 days of age. LH was undetectable in the male before 12 days of age, rose to a peak (440 plus or minus 60 (S.D.) ng/ml) at 13 days of age and fell below the detection level again between 15 and 25 days of age. A further increase then occurred which almost reached adult levels. LH was first detectable in the female rat at 11 days of age with a peak value of 130 plus or minus 35 ng/ml at 12 days. The hormone was undetectable on days 14 and 15, rose to a second peak on day (148 plus or minus 56 ng/ml), and was again absent between 19 and 25 days of age. The concentration rose, as in the male, between days 25 and 28 to a level similar to that of the adult. The results show sexual differences in prepubertal gonadotrophin surges. The LH peak at 12-13 days in both sexes appears to be light-dependent. The FSH peak at this time was affected by light but was not strictly light-dependent.