The responses of gastric inhibitory polypeptide (GIP) and insulin to intraduodenal and IV triglyceride infusions were measured in 11 late fetal and 10 neonatal pigs. Basal plasma glucose, insulin, and GIP concentrations were lower in fetal than in neonatal pigs. In the fetal pigs, plasma glucose increased slightly during intraduodenal and IV triglyceride infusions, whereas plasma insulin remained unchanged during the tests. No significant changes were observed in plasma GIP concentration following intraduodenal triglyceride infusion in the fetal pigs, but plasma GIP fell during the IV infusion of triglyceride in these pigs (p less than 0.01). In the neonatal pigs, plasma glucose and insulin remained unaffected by intraduodenal and triglyceride infusions. Plasma GIP did not change during the IV triglyceride infusion, but exhibited a paradoxical decline after the intraduodenal triglyceride infusion (p less than 0.05). It is concluded that the GIP-cell response to an oral triglyceride load is suppressed in late fetal and neonatal pigs. The abolished GIP response to oral triglycerides could play a causal role in the inactivity of the enteroinsular axis which is seen in both human and animal neonates.