Rat pancreatic tissues were fixed with our modification of Dalton’s fixative and the islets from various pancreatic regions were examined by electron microscopy. In addition, the effect of the alloxan-diabetic state on the pancreatic polypeptide cell was examined at the ultrastructural level. The results are as follows. (1) The islets of the rat contain four types of glandular cells: A, B, D and PP cells. (2) The PP cell is characterized by approximately 170-mµ-sized secretory granules with a rather ill-defined core. The ultrastructure of this type cell is described in some detail. (3) In the pancreatic portion which hooks around posterior to the superior mesenteric vessels, PP cells are abundant and some of them are markedly degranulated. Their cell organelles, such as the nucleolus, granular endoplasmic reticulum and Golgi apparatus are well developed. (4) In alloxan-diabetic rats, B and PP cells show disappearance and numerical increase, respectively. In the PP cells, hypertrophy of the Golgi apparatus and appearance of extremely electron-translucent secretory granules are not