Aims/hypothesis. Studies on the functional and morphological relations between exocrine and endocrine pancreas have been conducted mainly to disclose the influence of islets of Langerhans on acinar parenchyma. Less attention has been paid to the relations occurring between islets and pancreatic ducts. Methods. A series of consecutive sections of normal adult rat pancreas were double stained with islet (hormones) and duct (cytokeratin 20) markers. Electron microscopy was conducted to investigate the ultrastructural features of duct-islet relations and anti-insulin immunogold labelling was carried out to reveal the presence of insulin in the pancreatic duct system. Results. Consecutive double-stained sections demonstrated that 73.60 ± 2.97 % of the islets were attached to the ducts. For each series, 93.48 ± 5.43 % of the islets contacting the duct tree were associated with small-sized ducts or centroacinar cells. Electron microscopy revealed that some insulin and somatostatin cells do face the duct lumen. Insulin was detected within the duct lumen and in the endosomal compartment of the duct cells. Conclusions/interpretation. The finding that most islets are connected with the duct system in the adult pancreas is discussed in terms of hormone secretion into the ducts, islet histogenesis and the relation among the three tissue components of the pancreas, the endocrine, the exocrine and the duct system. [Diabetologia (2001) 44: 575–584]