Light microscopical immunohistochemistry was used to demonstrate the regulatory peptides present in the endocrine pancreas of Protopterus aethiopicus. The peptides studied included insulin, glucagon, pancreatic polypeptide and somatostatin. The results showed that the 4 regulatory peptides commonly detected in the mammalian endocrine pancreas were immunologically discernible in this dipnoan fish. Large amounts of insulin-immunoreactive cells, in the centre of the pancreatic islets, were surrounded by a small rim of glucagon-or pancreatic polypeptide-immunoreactive cells. In addition, adjacent sections stained with anti-glucagon and anti-pancreatic polypeptide revealed that these hormones could be found in the same cells. Somatostatin-positive cells were scattered throughout the islets. Their processes were seen to contact many different endocrine pancreatic cells, suggesting that the somatostatin-immunoreactive cells control the functions of other endocrine pancreatic cells.