The postnatal maturation and the adaptational ability of the sympathoadrenal system has been investigated in preterm neonates (n = 8), and in sick preterm neonates with respiratory disorders (n = 10). Plasma levels of dopamine (DA), norepinephrine (NE), epinephrine (E) and 3-4 dihydroxyphenylacetic acid (DOPAC) were evaluated at rest during the first month of life, and following an inhalation of a 5% carbon dioxide-21% oxygen mixture for 10 min. During the first month of life the sick preterm neonates exhibited similar NE, E, and DOPAC plasma levels but higher DA amounts than healthy infants. Plasma DA levels were inversely correlated with the transcutaneous oxygen tension (r = -0.636) indicating that hypoxemia was able to enhance the release of DA. Immediately following the hypercarbia test, there were no significant changes of plasma catecholamine levels in the sick preterms, but there was a significant increase of E plasma levels (+140%, p less than 0.05) and a moderate elevation of NE and DA amounts in the healthy preterms. It is concluded that preterm neonates who have had respiratory disorders did not exhibit an immaturity of the sympathoadrenal system at rest, but had a defect in the release of E following hypercarbia exposure, which may be secondary to an alteration in chemoreceptor function and/or reduced catecholamine stores.