An ultrastructural study of the early effects of intraperitoneal injection of 50 mg/kg body weight of N-nitrosomethylurea on the pancreatic islet tissue in Chinese hamsters was performed. Thirty minutes after injection there was slight dilatation of the endoplasmic reticulum and mitochondrial swelling in a few peripheral alpha2-cells near to capillaries and also in some beta-cells. After 1 hour these changes were more pronounced. After 3 hours, distinct nuclear pyknosis was seen in endocrine islet cells of all types together with beta-cells granular irregularities, disruption of granular cores and autophagy of alpha2-cell granules. During the following hours, marked derangement of intracellular structures, ending in obvious cellular destruction, was observed. The results indicate that N-nitrosomethylurea has a direct cytotoxic effect on the beta-, alpha2- and alpha1-cells of the islet tissue. The similarity between the ultrastructure of the pancreatic islets after injection of N-nitrosomethylurea and streptozotocin is discussed.