The gonads are usually considered quiescent organs in infancy and childhood. However, during the first few postnatal months of life, levels of gonadotropins and sex hormones are elevated in humans. Recent epidemiological evidence suggests that environmental factors operating perinatally may influence male reproductive health in adulthood. The early postnatal activity of the Sertoli cell, a testicular cell type that is supposed to play a major role in sperm production in adulthood is largely unknown. Recently, the peptide hormone inhibin B was shown to be a marker of Sertoli cell function in the adult male. In the adult woman, inhibin B is secreted by the granulosa cells. Longitudinal serum levels of inhibin B were measured in healthy boys (n = 15) and girls (n = 15), in cord blood, and every third month during the first 2 yr of life. In addition, serum levels of FSH, LH, and testosterone (boys) were measured in the same group of children. In boys, inhibin B, FSH, LH, and testosterone levels were all elevated at 3 months of age. However, the peak of inhibin B was unexpectedly high, into the supraadult range (mean +/- SE, 378 +/- 23 pg/mL) and persisted much longer than the elevation of FSH, LH, and testosterone. Thus, although levels of FSH, LH, and testosterone decreased into the range observed later in childhood by the age of 6-9 months, serum inhibin B levels remained elevated up to at least the age of 15 months. In girls, the hormonal pattern was generally more complex, with a high interindividual variation in levels of inhibin B, FSH, and LH within each age. In conclusion, the sustained elevation of inhibin B to supraadult levels in infant boys indicates that the neonatal period may be a developmental window important for Sertoli cell proliferation and maturation. Thus, the gonads may be potentially vulnerable to exogenous endocrine interference, e.g. from environmental factors during this period of life. Measurement of serum levels of inhibin B in infants may give clinical clues about developmental deficiencies in the gonads that otherwise only become apparent around puberty or later in life.