Following the intraduodenal installation of purified 125I-labeled human trypsin up to about 4--6% of the label was measured after 15--30 min in blood plasma and found to separate in a dextran-gel filtration system similar to purified human trypsin (-125I) after incubation with human serum. About 1% of the installed trypsin-(-125I)-dose was found already after 20 min in 100 ml of aspirated pancreatic secretion and later on also in the duodenal content. The results support the concept of the existence of an enteropancreatic circulation of trypsin also in man and explain in part the low to non-detectable levels of immunoreactive serum trypsin observed in patients with exocrine pancreatic insufficiency.