Cortisol, cortisone, corticosterone, corticosterone sulfate and 11-deoxycorticosterone sulfate were partly purified by paper chromatography from plasma aliquots obtained from the umbilical cord, from premature newborn infants with and without respiratory distress syndrome, and from normal adults, and quantified by competitive protein binding assays. The results emphasize the predominance of cortisone (13.6 ± 3.6 μg per 100 ml) over cortisol (8.6 ± 3.7 μg per 100 ml), and the quantitative significance of the sulfates of corticosterone (4.4 ± 1.9 μg per 100 ml) and 11-deoxycorticosterone (6.4 ± 2.4 μg per 100 ml) in umbilical cord plasma. On the 1st, 3rd and 5th days of life the plasma concentrations of corticosterone sulfate are not statistically different from those of umbilical cord plasma, while on the 3rd and 5th days those of 11-deoxycorticosterone sulfate fall to about half the umbilical cord plasma values for this conjugate (p ≃ 0.005). Premature infants whose gestational age ranged from 28 to 36 weeks and who suffered from respiratory distress syndrome (24 observations), presented an elevated plasma concentration of cortisol (p ≃ 0.01), corticosterone (p ≃ 0.01) and corticosterone sulfate (p ≃ 0.005) compared to control premature infants of comparable gestational age (10 observations). In adult subjects the plasma corticosteroid pattern is characterized by the predominance of cortisol. All other steroids measured, with the exception of cortisone, were detected in concentrations less than 0.5 μg per 100 ml. The data emphasize the usefulness of specific cortisol measurements as an index of adrenocortical function during early postnatal life.