The ultrastructural characteristics of immunoreactive enteroglucagon cells from three groups of experimental rat models (i.e., massive small bowel resection alone, massive small bowel resection with bombesin infusion, and transection with bombesin infusion) have been investigated. The results show that the cells did not undergo significant change in size or in the area of cytoplasm. All three groups displayed highly significant decreases of between 200% and 400% in the area of cytoplasm occupied by the secretory granules (expressed as volume density) and highly significant increases between 150% and 200% in the area occupied by the rough endoplasmic reticulum. Significant decreases (7%-10%) in secretory granule diameter were found in the two bombesin-treated groups, whereas resection alone was without significant effect on granule size. These results indicate that the enteroglucagon cells in these three groups are in a hyperfunctional state.