Guinea pigs were maintained hyperglycemic and glucosuric by daily subcutaneous injections of prednisolone suspension. The stimulated beta cells showed de granulation, increased density of rough endoplasmic reticulum and expanded elaborate Golgi complexes. Glycogen particles accumulated progressively in these cells until by the third month glycogen aggregates encroached on large areas of the cytoplasm. By the fourth and fifth month many beta cells appeared as small bags filled with glycogen. Cytoplasmic remnants in these glyco gen-filled beta cells showed organelles of normal appearance. Beta cells proliferated during the first two months as evaluated by the proportion of beta cells incorporating 3H-thymidine. In guinea pigs killed after threeand four months of prednisolone treatment the 3H-thymidine index declined to levels found in untreated guinea pigs.