The course of alloxan diabetes was studied ultrastructurally during 19 months in duct‐ligated and control rats. During the first days after alloxan administration, selective degeneration was observed in most β‐cells of both atrophic and intact pancreatic parenchyma. New formation of islet cells was found from about one week. Two different kinds of cells without characteristic granulation were seen: those which seemed to be precursor cells to granulated islet cells, and those which appeared to represent hypertrophic, de‐granulated β‐cells. The number of α2‐cells was increased during the first months after the injection of alloxan. At 19 months a normal fine structure was found i