The effect of transpyloric and nasogastric feeding on the blood concentration of glucose, lactate, pyruvate, glycerol, hydroxybutyrate, acetoacetate, alanine, insulin, pancreatic and total glucagon was determined in 20 preterm infants. The babies were studied on the last day of transpyloric feeding and the first and fifth days of ensuing nasogastric feeding. In 9 infants hourly measurements of hormones and metabolites were made at 1000, 1100, 1200, 1300 and 1400 hours. The blood concentrations of glucose, alanine, pancreatic and total glucagon were stable, the concentration of the other metabolites and insulin, less so. No significant difference in mean metabolite or hormone concentration was noted by time of day or type of feeding. Measurements made on the fifth day of nasogastric feeding showed no significant differences from those at the time of changeover. The infants were clinically well and growing normally at the time of study, but had low plasma insulin and high plasma glucagon concentrations. We conclude (i) the site of presentation of milk in the gastrointestinal tract has no effect on the circulating concentration of selected metabolites and hormones in the preterm infants, (ii) the preterm infant grows at a normal rate with a plasma insulin/glucagon ratio that in the adult would be expected to favour catabolism.