The response of the thoracic aorta to balloon-catheter-induced endothelial denudation was studied in two animal models of diabetes: the obese Zucker rat and the streptozotocin-treated Wistar rat. The obese Zucker rat, an animal with a metabolic profile similar to that of noninsulin-dependent human diabetics, was characterized by excessive body weight, hyperinsulinemia, hyperlipidemia, and a mild increase in plasma glucose levels. In Zucker rats sacrificed 3 weeks after endothelial denudation, the cross-sectional areas of the fibrocellular intimal lesions were found to be approximately twice as large as those of the lean control. In all other respects, the morphology of the lesions in the obese rats, as assessed by both light and transmission electron microscopy, was similar to that of the lean control rats. In streptozotocin-treated Wistar rats, neither the cross-sectional areas nor the morphology of the intimal lesions differed from those in control rats. These results indicate that, in the obese Zucker rat, the response to aortic intimal injury is altered.