Previous studies have suggested that both newborn animals and humans absorb intact proteases from the intestine in greater amounts than do adults. Absorbed proteases are rapidly complexed in plasma by several inhibitors which inactivate free proteases. In order to confirm increased intestinal uptake in newborns, we evaluated the plasma trypsin activity in both 2-week-old and adult rats following a trypsin feed. In addition, we studied the interaction between absorbed trypsin and rat trypsin inhibitors (α-macroglobulin, α₁-inhibitor 3, and α₁-protease inhibitor) by determining the concentration of α-macroglobulin complexes both in vivo and in vitro following trypsin feeding and by evaluating the amount of trypsin activity complexed with the different inhibitors. In vivo experiments demonstrated that trypsin uptake was significantly increased in newborns compared to adult rats and that 50–70% of plasma trypsin activity was associated with αi-inhibitor 3. Increased uptake was not accompanied by increased α-macroglobulin complexes. In vitro trypsin incubation did not lead to increased α-macroglobulin complexes until the other inhibitors were removed. These findings suggest that newborns have increased trypsin uptake, and the trypsin initially binds to α₁-inhibitor 3 before being transferred to α-macroglobulin for clearance. Further studies are needed in order to understand the interaction between intestinal uptake and plasma inhibition of absorbed proteases.