The present article describes an unusual proliferative islet finding observed incidentally in a young male Wistar rat in a 2-week toxicity study. Histologically, the islet lesion was characterized by diffuse enlargement of the islets, which consisted of peripheral proliferation of non-insulin-containing islet cells surrounding normal-appearing insulin-containing cells in the center. To the authors' knowledge, this is the first report of spontaneous proliferative islet lesion composed of non-insulin-containing cells in young rats.