Hydrolysis of heptadecapeptide gastrin (G-17) by endopeptidase 24.11 (EC 3.4.24.11) was studied in vivo and in vitro in the pig. Ion exchange chromatography and radioimmunoassay with three region-specific antisera were used to identify the products of porcine G-17 degradation. Incubation of antral extracts with pure endopeptidase 24.11 resulted in a substantial loss of intact G-17: 80% C-terminal immunoreactivity was lost in 60 min. This hydrolysis was completelyinhibited by phosphoramidon, which is a specific inhibitor of endopeptidase 24.11. In antral extracts G-17 accounted forgreater than 95% of total C-terminal immunoreactivity, compared with less than 60% C-terminal immunoreactivity in the gastric venous outflow; shorter C-terminal forms comprised the major part of the remaining immunoreactivity. After infusion of phosphoramidon, the concentration of intact G-17 was increased, and there was a corresponding reduction in the concentration of other C-terminal immunoreactive fragments. We conclude that endopeptidase 24.11 degrades G-17 in vitro and in vivo and may be responsible for the generation of C-terminal fragments from G-17 after secretion from the porcine antral mucosa.