TUNEL staining, which allows detection of fragmented DNA in situ, is commonly used as an indication of apoptosis. Recent studies suggest that apoptosis is increased in several pathophysiological conditions. In this study we examined the hypothesis that chronic diabetes is associated with an increase in TUNEL staining of the aorta. Diabetic rats were studied 4-5 months after injection of streptozotocin (50 mg/kg). Aorta of diabetic and control rats were examined for TUNEL staining, morphology by electron microscopy, and DNA contamination in RNA preparation by polymerase chain reaction (PCR). TUNEL staining of aortic sections showed a 6 fold increase of positive cells in the media of diabetic aorta (22 +/- 6%) (mean +/- SE) compared with aorta from age-matched controls (3.6 +/- 0.9%, p < 0.05). Electron microscopy demonstrated typical apoptotic cells and bodies in the media of aorta from diabetic but not control rats. DNA contamination was found in RNA prepared from diabetic aorta, which was detected using PCR, which is consistent with increased DNA fragmentation. Increased TUNEL staining was not observed in rats with hyperglycemia 3 days after injection of streptozotocin. In conclusion, severe chronic diabetes is associated with an increase in TUNEL staining, and perhaps apoptosis, in the aorta. We speculate that increased apoptosis may compensate for increased proliferative activity in diabetic blood vessels.