The adrenal function of 10 preterm infants of gestational age 27-34 weeks was assessed longitudinally from measurements of excretion rates of steroid metabolites in 24-hour urine samples collected at frequent intervals for up to 80 days after delivery. The changes in steroid excretion with time in preterm infants of gestation over 28 weeks reflect involution of the fetal adrenal zone at a similar rate to term infants. These findings are consistent with the removal at birth of the inhibitory effects of oestrogen on the 3 beta-hydroxysteroid dehydrogenase enzyme. The continued function of the adrenal fetal zone beyond the first month in preterm infants of less than 28 weeks gestation may however be due to persistence of some other fetal regulatory adrenal mechanism.