The exocrine and endocrine pancreas was investigated according to the fluorescence histochemical method of Flack and Hillarp. 1) Green fluorescent adrenergic fibers were regularly seen associated with arteries and arterioles in the exocrine pancreas. 2) Cholinergic fibers as shown by cholinesterase activity, were also found in the parenchyma of pancreas. 3) Yellow fluorescent cells scattered in the exocrine parenchyma and localized to a population of pancreatic islet cells with a characteristic distribution at the islet periphery was found. 4) By the fluorescence microscopic observation, inter-or intralobular pancreatic ducts, involving the zymogen granules, can also be seen after treatment with HCL vapor. 5) Yellow fluorescent cells, beta-cells containing insulin, remained at the Islet periphery. At present, the above mentioned yellow fluorescent cells are identified as containing HPP (Human pancreatic polypeptide) according to the immunofluorescence technique. With the use of the Falck and Hillarp histochemical technique ethionine induced pancreatitis in cats has been investigated. 1) After seven days of ethionine (5 mg/kg BW oral ad.) treatment, pancreas showed histochemical changes such as hemorrhage, fat necrosis, destruction of acinar cells and degranulation of zymogen from the parenchyma of pancreas. 2) Oral administration of ethionine for ten days induced severe degranulation, rupture of vessels, especially of veins and venules and later influenced arteries or arterioles. 3) Necrosis and fibrosis began to appear in the spaces between the cellular debris and marked pancreatic atrophy could be found. 4) The destruction of Islets of Langerhans can be found in the ethionine induced pancreatic parenchyma. On the other hand, an increased number of Islets of Langerhans was also observed in the site of lobule. 5) The presented findings may also suggest that the duration of administration of ethionine is more important factor than graded doses of ethionine in the production of ethionine is more important factor than graded doses of ethionine in the production of ethionine induced pancreatitis in cats.