Seventeen Wistar inbred rats were made diabetic by a single injection of streptozotocin (50 mg/kg body weight) and were killed after periods of 3, 6, 9 or 12 months. Pathological changes in pericapillary Müller cells of the retina were studied using electron microscopy. Basement membrane-like material proliferated in the intercellular space of the Müller cell network and occasionally appeared to insinuate into the Müller cell cytoplasm far from capillary pericytes and endothelial cells. The part of the Müller cell that was enveloped by proliferating basement membrane-like material showed partial necrosis which was thought to contribute to the widening of the capillary wall. A breakdown of the retinal framework, which leads to capillary dilatation, was also thought to be associated with partial necrosis of Müller cells. Highly electron-dense bodies accumulated in the Müller cell cytoplasm which surrounded the retinal capillaries. Ultrastructurally, these dense bodies resembled lysosomes. Their increased number might reflect the altered metabolism of the diabetic retina.