The study aimed to determine the developmental changes in the response of peripheral and visceral tissue protein synthesis to feeding during early postnatal life and the associated changes in circulating insulin, insulin-like growth factor (IGF-I), and amino acid concentrations. Tissue protein synthesis was measured in vivo with a large dose of L-[4(-3)H]phenylalanine in 7- and 26-day-old pigs that were either fasted for 24 h or refed for 2.75 h after a 24-h fast. Fractional rates of protein synthesis (Ks) in skeletal muscle, heart, and liver were greater in 7-than in 26-day-old pigs. Refeeding stimulated Ks in skeletal muscle, pancreas, jejunum, and liver of both 7-and 26-day-old pigs. The stimulation of skeletal muscle and jejunal Ks by refeeding was greater in 7- than in 26-day-old pigs. Plasma IGF-I concentrations were lower in 7- than in 26-day-old pigs. Plasma concentrations of insulin and amino acids increased with refeeding. The increase in plasma insulin concentrations with refeeding was greater in 7- than in 26-days-old pigs. These results indicate that the stimulation in skeletal muscle and jejunal protein synthesis by feeding is elevated in young compared with older suckling pigs. This enhanced stimulation of protein synthesis by feeding in neonatal pigs is associated with elevated circulating concentrations of insulin but not amino acids or IGF-I.