During the neonatal period, adrenal steroidogenic function was assessed by determining the peripheral levels of eight steroids by radioimmunoassay with the use of specific antisera. Serum concentrations of cortisol and cortisone in the umbilical artery were 92.2 +/- 13.3 ng/ml (m +/- SE) and 78.0 +/- 11.1 ng/ml, respectively, and these levels decreased to 37.4 +/- 8.2 and 47.9 +/- 14.7 ng/ml on the first day after birth. Serum concentrations of progesterone, 17 alpha-hydroxyprogesterone, total pregnenolone, and total dehydroepiandrosterone in the umbilical artery were 294 +/- 41, 8.7 +/- 1.0, 611 +/- 86, and 867 +/- 179 ng/ml (mean +/- SE), respectively, and decreased immediately after birth. In contrast, serum levels of total 16 alpha-hydroxydehydroepiandrosterone in the umbilical artery were 3,415 +/- 416 ng/ml, and those levels were maintained on the first day of life, whereas serum levels of 16 alpha-hydroxypregnenolone in the umbilical artery were 667 +/- 173 ng/ml and increased to 5,180 +/- 728 ng/ml. The significance of these changes in adrenal steroids during the neonatal period is discussed in the text.