Plasma concentrations of 17α-hydroxyprogesterone (OHP) were measured by a specific and sensitive radioimmunoassay in 348 blood specimens of male and female infants from birth to 2 years of age. Total corticoids levels were assayed by a competitive protein binding technique.OHP levels were higher in the cord than in the peripheral blood of both the mother and the newborn. At term OHP cord levels were higher in female than in male infants. Mean peripheral concentrations of OHP were 419 ± 217 in boys and 431 ± 175 ng/dl in girls in the first day of life and decreased during the first week of life to 70 ± 37.5 and 95 ± 39 ng/dl in male and female infants, respectively. Thereafter, a sex-linked difference in the pattern of OHP plasma concentrations was observed during infancy and, moreover, this dimorphism varied with age, values being higher in boys at 1–2 months but higher in girls at 6–9 months of age. Total corticoids values did not vary significantly with either sex or age.The pattern of OHP in boys closely parallels that which we described previously for testosterone and further documents the postnatal endocrine activity of the testis. The pattern of OHP in girls suggests that in human infant the ovaries exhibit a moderate and rhythmic endocrine activity which lasts at least until the end of the first year of life.SpeculationIt has been shown that in human infant the testis has a marked and transient (for about 4 months) endocrine secretion (OHP and testosterone) after the postnatal activation of the hypothalamopituitary function. In contrast, in female infants, the ovaries exhibit a moderate, rhythmic, but more prolonged endocrine secretory activity (OHP and estradiol). This could result from a pulsatile secretion with great amplitudes but a low tonic gonadotrophin activity in infancy. It is speculated that in the ovary a more sustained luteinizing hormone (LH) (or LH + follicle-stimulating hormone (FSH)) stimulation could be necessary to achieve full maturation of its steroidogenic enzymes, in particular, of the aromatase.