Adrenal steroidogenesis was evaluated in 25 sick premature infants with a gestational age of less than 30 weeks. ACTH stimulation tests were performed on the fourth day of life using synthetic ACTH (36 micrograms/kg). Considering the stress and degree of illness, preterm newborns had low basal cortisol levels (mean +/- SEM, 207.4 +/- 23.5 nmol/L), and their levels were similar to basal levels reported for healthy full-term newborns (170.7 +/- 26.8 nmol/L; P = 0.31; reference data from Endocrine Sciences, Inc., Calabasas Hills, CA). However, compared to term neonates, preterm infants had markedly elevated basal levels of 17-hydroxypregnenolone (54.3 +/- 11.2 nmol/L), 17-hydroxyprogesterone (19.7 +/- 4.0 nmol/L), and 11-deoxycortisol (19.1 +/- 3.3 nmol/L), which were 7-, 18-, and 8-fold higher, respectively, than values for term infants. The activity of 3 beta-hydroxysteroid dehydrogenase was not significantly reduced in extremely premature neonates (mean basal ratio of 17-hydroxypregnenolone/17-hydroxyprogesterone, 2.9 +/- 0.2; ACTH-stimulated ratio, 6.5 +/- 0.4). In contrast, the mean basal substrate/product ratio of 11-deoxycortisol was markedly elevated in the preterm infants (11.9 +/- 2.2, ratio x 10(-2) compared to that in the full-term infants (2.1 +/- 0.4, ratio x 10(-2); P < 0.001). These findings are consistent with decreased activity of 11 beta-hydroxylase (11 beta OH) in preterm infants born at less than 30 weeks gestation. Decreased 11 beta OH activity appears to be more prominent than the deficiency of 3 beta-hydroxysteroid dehydrogenase that has been found in infants with lesser degrees of prematurity, suggesting that 11 beta OH activity may be regulated during fetal development to increase during the latter part of gestation.