Plasma pancreatic polypeptide (PP) concentrations have been studied in 197 healthy term and preterm infants. At birth plasma concentrations were lower than those found in the young fasting adult (p less than 0.01), but by the sixth postnatal day in both term and preterm infants basal concentrations had risen to adult levels. In preterm infants, who were studied at two further postnatal ages, 13 and 24 days, basal plasma PP concentrations peaked at 13 days with levels that were over twice those seen in fasting adults (p less than 0.005). However, the marked PP elevations following feeding that have been reported in adults, were not seen in any of the groups of neonates studied. PP physiology thus appears to differ in neonates and adults. These findings may be relevant to adaptation to postnatal life.