Using sensitive radioimmunoassays we have measured and compared plasma concentrations of motilin, gastrin, enteroglucagon, neurotensin, gastric inhibitory polypeptide and pancreatic polypeptide in (a) 53 healthy, preterm infants at birth or preprandially at 2.5, 6, 13 or 24 days; (b) 45 normal, breast-fed, term infants at birth or preprandially at 6 or 16 days, and (c) 12 healthy fasting adults. Plasma concentrations of all six hormones rose during the neonatal period in both preterm and term infants, the first four of these hormones reaching levels which exceeded those seen in healthy fasting adults. The rate of increase and the magnitude of the changes were less in term infants than preterm infants. These changes in plasma hormone concentrations may be the result of enteral feeding. Gut hormones exert important effects on gut growth, secretion and motility and on intermediary metabolism, and the postnatal hormonal surges observed may play a key role in the postnatal adaptions to enteral feeding.