Plasma levels of aldosterone, corticosterone, 11-deoxycorticosterone, progesterone, 17-hydroxyprogesterone, 11-deoxycortisol, cortisol, and cortisone were measured simultaneously by a micromethod of multisteroid analysis in eight vaginally delivered premature infants (PI) of 33-36 wk gestation with uneventful peri- and postnatal course. Mean concentrations (ng/ml) in umbilical arterial and in peripheral venous or capillary plasma sampled longitudinally at age 2 h to 7 days were compared with the same kind of data obtained from a group of 12 term infants (TI) who served as controls. Mean aldosterone was two to five times higher in PI than in TI (umbilical artery, 2 h to 7 days; p less than 0.05), whereas 11-deoxycorticosterone was lower in PI from 2 h (p less than 0.01) until 7 days (NS). Corticosterone was significantly higher in PI than TI at 6 and 24 h after birth, whereas cortisol was slightly lower (NS) in PI in umbilical artery and 2 h after birth, but higher (p less than 0.02) at 6 h, showing less variation in PI than in TI. 17-Hydroxyprogesterone levels in PI were two to three times higher (p less than 0.02) during 6 h until 7 days after birth. The data suggest that PI are able to maintain high aldosterone levels in the early neonatal period. Higher levels of the active glucocorticoids (cortisol and corticosterone) seen after delivery point to a more stressful extrauterine adaptation of PI. Furthermore, the data demonstrate that the adrenal cortex is fully functioning in premature infants (33-36 wk gestation) as well as in term infants.