This study was undertaken to determine the differences (if any) in cerebral blood flow (CBF) between streptozotocin (STZ) diabetic and normal rats. CBF was studied in connection with episodes of hypoxia, hypercapnia and hypotension as compared to the basal condition. Overall basal CBF rates in streptozotocin diabetic rats were significantly higher than in normal animals. However, initial basal flow rates prior to the first challenge were insignificantly higher in the STZ diabetic group. The higher CBF rate in STZ diabetics was also seen during the peak flows of the hypoxic and hypercapnic challenges. Furthermore, although overall CBF decreased for both the normal and STZ diabetic groups during hypotension, higher CBFs were observed in the STZ diabetic group during this challenge. The percent increase in CBF above control resulting from hypoxia or hypercapnia and the changes in CBF resulting from hypotension were not significantly different in the STZ diabetic and normal groups. The results indicate that the STZ diabetic rat regulates CBF in the same manner as the normal rat in response to hypoxia, hypercapnia and hypertension. The STZ diabetic rat executes these CBF responses at a slightly higher CBF rate. In view of the finding that the regulation of CBF is unaltered in the STZ diabetic animal, it is hypothesized that the associated hyperglycemia may be the causative agent for the cerebral ischemic susceptibility associated with long-term diabetes mellitus rather than a failure of CBF regulation.