BACKGROUND Recent experimental findings have suggested that activation of trypsinogen by cathepsin B within acidic pancreatic acinar cell cytoplasmic vacuoles may be a critical early event in both secretagogue and diet-induced pancreatitis. The weak base chloroquine accumulates within acidic intracellular compartments, raises their pH, and can inhibit proteolysis as well as cathepsin B. METHODS We have investigated the effect of in vivo chloroquine administration on both secretagogue and diet-induced experimental pancreatitis to determine if raising the pH of cytoplasmic vacuoles in these models of pancreatitis would have a protective effect. RESULTS Infusion of chloroquine (5 mg.kg-1.h-1) resulted in the uptake and concentration of chloroquine in the pancreas, an increase in the pH of acinar cell acidic compartments, and interference with the pH-dependent sorting of lysosomal hydrolases from digestive enzyme zymogens. However, chloroquine administration did not have a protective effect against the hyperamylasemia, the pancreatic edema, the morphological changes or the mortality that is associated with these models of pancreatitis. CONCLUSIONS These observations lead us to conclude that raising the pH of acinar cell acidic compartments by in vivo administration of chloroquine does not prevent either secretagogue or diet-induced pancreatitis.