Serum dehydroepiandrosterone sulfate (DHAS) was measured by radioimmunoassay in blood samples obtained in 128 ill newborn infants. Serial sampling was carried out in 40 infants. There were wide ranges found in the values in all gestational age groups, and there were not significant differences in the first day of life between DHAS levels in less than 30 week gestation prematures, 6819 +/- 4631 (SD) ng/ml, and near term or term infants, 4307 +/- 1498 ng/ml. Mean DHAS concentrations did not decline over the first three weeks of life in prematures less than 36 weeks gestation. In six infants, age 35-73 days, and 29-34 weeks gestation at birth, the DHAS concentration was 1068 +/- 138 ng/ml. High concentrations were frequent in prematures less than 33 weeks gestation and could be correlated to epiodes of severe clinical stress. There were no significant differences in serum DHAS levels, on the first day of life, between infants with no hyaline membrane disease, nonfatal hyaline membrane disease and fatal hyaline membrane disease. Intrauterine growth retarded (IUGR) infants, who were greater than 35 weeks gestation, had significantly lower (P less than .032) DHAS levels in the first day of life than normally grown infants. The results show that there is a persistence during the postnatal period of the prominent delta5-3beta-hydroxysteroid production by the adrenal cortex characteristic of the fetus. Low concentrations of serum DHAS in IUGR infants suggest that the fetal zone of the neonatal adrenal cortex is a major source of circulating DHAS in the newborn period.