The effect of insulin treatment on the exocrine pancreas of streptozotocin-diabetes rats was investigated by light and electron microscopy. In the diabetic rats treated with daily lente insulin injection for four weeks, the islets became hyperplastic and proliferative, although degenerative and atrophic islets caused by streptozotocin remained if diabetic rats were not treated with insulin. Fibrosis and degeneration of the acinar cells were not found in all the diabetic rats by light microscopic examinations. Electron microscopic examinations showed that acinar cells of the exocrine pancreas of the diabetic rats without insulin treatment were characterized by irregular dilatation and prominent lamellar arrangement of rough endoplasmic reticulum and by nuclear pyknosis. After one hour of a single injection of regular insulin, rough endoplasmic reticulum of the acinar cells of the diabetic rats was rapidly activated and many intracisternal granules appeared. When the daily injection of insulin was continued, the acinar cells became to show regular arrangement of rough endoplasmic reticulum, much less vacuolarizations and less immature zymogen granules in comparison with those of the untreated diabetic rats. Exocrine pancreas of the insulin-treated rats revealed a lot of autophagic vacuoles which were supposed to derive from lysosomes. These results suggested that insulin had a repairing effect on the damaged acinar cells in diabetic state.