We studied suckling rats from 1 to 28 d of age to determine the normal developmental changes in the protein synthetic activity of gastrointestinal tissue. We also studied the changes that occurred in response to 10 or 18 h of fasting at 5, 10, 16 and 28 d of age. Protein synthesis was measured in vivo using a flooding dose of L-[4-3H]phenylalanine. Fractional protein synthesis rates (FSR, %/d) of stomach and small intestines increased nearly 80% during the period normally associated with weaning (d 18 to 28). Between birth and 28 d, the pancreas FSR increased approximately 6.5-fold from 46 to 302%/d. The increases in stomach, small intestinal and pancreatic FSR were largely due to increases in translational efficiency (gram protein synthesized per gram RNA). At 5, 10, 16 and 28 d postpartum, both the FSR and translational efficiency in pancreatic and small intestinal tissues were decreased after 10 h of fasting; however, measures in stomach tissue were largely unaffected. The magnitude of decline in FSR and translational efficiency in response to fasting was greater during the earlier than in the later stages of development. The results suggest that the FSR in the pancreas, small intestines and stomach of suckling rats increase during the period of development associated with weaning. Fasting affected protein synthetic activity more profoundly in pancreatic and small intestinal tissue in the early stages of development.