Our previous study on phase II detoxifying enzymes, showing a significant reduction of glutathione S-transferase-pi in chronic pancreatitis compared to the normal pancreas, indicated that xenobiotic-metabolizing enzymes are involved in the pathogenesis of pancreatic diseases. This study presents an overall look at the distribution of the phase I xenobiotic-metabolizing enzymes, which are responsible for the metabolism of many common environmental toxins and carcinogens, in the normal pancreas. Twenty-four normal pancreases from 7 donors and 17 early autopsy cases, as well as cultured human islet cells, were analyzed by immunohistochemistry, Western blot analysis, and/or reverse-transcription polymerase chain reaction (RT-PCR) for the expression of nine cytochrome P-450 mono-oxygenases (CYP) and the NADPH cytochrome P-450 oxidoreductase. Remarkable differences in the cellular distribution of these enzymes were found between the individuals and between different pancreatic cells within the same individual. Nondiabetics expressed more of the enzymes than diabetics, females more than males, younger more than older individuals, and organ donors (all young individuals) more than autopsy specimens. CYP 2B6 was expressed in all 7 donor pancreas, compared to 8 of 17 autopsy cases. Most of the enzymes were localized in islet cells and either were distributed in all islet cells or were restricted to, or expressed in a higher concentration in, glucagon and/or pancreatic polypeptide cells. Furthermore, a different cellular localization of the enzymes was found in some individuals (e.g., cytoplasmic vs. Golgi pattern of staining and a frequent nuclear localization of CYP 2E1 in females). Except for anti-CYP 1A2 and 3A4, RT-PCR and Western blot analyses validated the specificity of the antibodies. Our results show that islet cells play a major role in the detoxification process of the pancreas. The expression of individual enzymes and their distribution in acinar, ductal, and islet cells may determine individual susceptibility to pancreatic diseases.