Multiple foci of hepatocytes differentiate in the pancreas of adult rats subjected to a copper depletion-repletion regimen. Copper deficiency for seven to nine weeks causes an irreversible depletion of over 80% of the acinar cells in the pancreas. When transferred to a normal diet, these rats exhibit only a minimal and spotty acinar cell recovery. This disruption of tissue organization appears to trigger a profound change in cellular commitment, which leads to hepatocyte differentiation in the "oval cells" in the periductal interstitium and the epithelial cells lining the small pancreatic ductules. Pancreatic hepatocytes express several liver-specific genes including albumin, a2u-globulin, carbamoylphosphate synthetase-I, and urate oxidase. Both carbamoylphosphate synthetase-I and glutamine synthetase, the ammonia-metabolizing enzymes, are expressed by all pancreatic hepatocytes; in liver, these are expressed by different populations of hepatocytes. The magnitude of hepatocyte differentiation in this model should facilitate studies on the molecular events regulating changes in cell lineage or differentiation commitment within the pancreas.