The endocrine cells of the processus uncinatus in the dog pancreas were investigated with special reference to the formerly known F-cell. The F-cell was detected frequently in the periphery of pancreatic islets as well as among exocrine tissue. In both localizations the F-cell shows similar ultrastructural features. Membrane-bound irregularly shaped secretory granules of variable electron density were seen. The cell possesses all features of an endocrine polypeptide secreting cell. Using the immunofluorescence and immunoperoxidase technique in the uncinate processus of the dog, we could reveal that the anti-sera against bovine pancreatic polypeptide (BPP) reacts with the cell which is localized at the same sites as the F-cell. We therefore conclude that the pancreatic F-cell is identical to the pancreatic polypeptide-producing cell. The other endocrine cell types of the dog pancreas are glucagon-producing A-cells, insulin-producing B-cells, and somatostatin-producing D-cells, as well as serotonin-producing EC-cells which are regularly present in the dog pancreatic islets and also scattered among exocrine tissue and the duct epithelial cells.