We measured FSH and LH concentrations in 130 cord sera and 217 peripheral sera obtained from infants of 28-42 weeks in gestation, aged 5-75 days after birth, by radioimmunoassay. Cord FSH and LH levels were 3.9-13.6 mIU/ml and 43.3-88.5 mIU/ml respectively, and decreased with advancing gestation. Although FSH levels in 28-31 weeks were higher in female than in male, another sex difference was not found. Postnatal FSH levels in male preterm infants decreased within 10 day, thereafter continued low levels (3.7-5.6 mIU/ml). However, in male term infants and female infants, FSH levels increased with peak levels between 11-30 days after birth, and then decreased gradually. The elevation of FSH levels in female was more remarked and prolonged in preterm infants than in term infants. Postnatal LH levels in both male and female decreased rapidly after birth, and then had similar pattern of FSH levels. Serum gonadotropin levels during the neonatal period were higher in female than in male, except for LH in term infants (male greater than female).