The intracellular activation of trypsinogen, which is both pH- and calcium-dependent, is an important early step in the development of acute pancreatitis. The cellular compartment in which trypsinogen activation occurs currently is unknown. We therefore investigated the site of intracellular trypsinogen activation by using an established cellular model of acute pancreatitis: supramaximal stimulation of pancreatic acinar cells with cholecystokinin. We used fluorescent dextrans as fluid phase tracers and observed the cholecystokinin-elicited formation and translocation of large endocytic vacuoles. The fluorescent probe rhodamine 110 bis-(CBZ-l-isoleucyl-l-prolyl-l-arginine amide) dihydrochloride (BZiPAR) was used to detect trypsinogen activation. Fluid phase tracers were colocalized with cleaved BZiPAR, indicating that trypsinogen activation occurred within endocytic vacuoles. The development of BZiPAR fluorescence was inhibited by the trypsin inhibitor benzamidine. Fluorescein dextran and Oregon Green 488 BAPTA-5N were used to measure endosomal pH and calcium, respectively. The pH in endocytic vacuoles was 5.9 ± 0.1, and the calcium ion concentration was 37 ± 11 μM. The caged calcium probe o-nitrophenyl EGTA and UV uncaging were used to increase calcium in endocytic vacuoles. This increase of calcium caused by calcium uncaging was followed by recovery to the prestimulated level within ≈100 s. We propose that the initiation of acute pancreatitis depends on endocytic vacuole formation and trypsinogen activation in this compartment.