Plasma secretin has been measured in 96 normal 6-day-old term infants and in 158 healthy preterm infants whose mean post-partum ages were 2 1/2, 6, 13 or 24-days. At birth, plasma secretin levels in both term and preterm infants were high compared with those seen in healthy fasting adults (p less than 0.001), but subsequently declined towards adult values. In contrast, preterm infants who had not been fed for the first 6 days of life, had presistently high basal plasma secretin values. In term infants at 6 days of age and in preterm infants up to 13 days, there was no secretin response to a feed. However, by 24 days, preterm infants showed a marked postprandial secretin elevation (p less than 0.02). No correlations were found between plasma secretin concentrations and either blood glucose or plasma insulin concentrations following a feed. Significant adjustments in plasma secretin levels occur in the early weeks of life which may be influenced by enteral feeding.