Hydrolyzed soy protein (SH) could be used as a protein source in formulas for infants with intolerance to cow's milk protein and may be preferable to intact soy protein (SI). However, metabolic responses to SH are poorly defined. Because of their partially hydrolyzed nature, nonphysiological elevations in either plasma amino acids or regulatory hormones may occur. Therefore, we evaluated effects of SH on plasma nutrient and pancreatic hormone (insulin, glucagon) concentrations. In Experiment 1, 24 newborn pigs were fitted with umbilical arterial and portal catheters, fed formula for 36 h and food deprived for 12 h. Pigs were then fed formula including either SH or SI with glucose polymers or casein-whey proteins (CW) containing lactose, and serial blood samples were taken for 2 h postprandially. Peak portal exceeded peak arterial amino acid concentrations within each treatment, and peak amino acid concentrations in CW-fed pigs exceeded those of SH- and SI-fed pigs. However, only SH formula-fed piglets had higher postprandial portal minus arterial amino acid concentrations (P < 0.05) throughout Experiment 1, suggesting that SH was well digested and absorbed. In Experiment 2, arterial catheters were inserted in 24 piglets. Previous procedures were followed except dietary carbohydrate was standardized to glucose polymers for all three diets, and sampling was extended to 3 h. Overall, portal or arterial nutrient and hormone concentrations were not different in the SI and SH groups (P > 0.05), indicating that hydrolyzed soy protein did not cause abnormal plasma concentrations. In conclusion, hydrolyzed soy protein did not result in elevated nutrient concentrations or hormone responses compared with intact soy or cow's milk protein.