To study whether transient ischemia is influenced by hyperglycemia, the middle cerebral artery was occluded for 5, 10 and 15 min in normo- and hyperglycemic rats. Five-minute ischemia induced minor lesions in both groups. After 10-min ischemia a significant greater infarct volume was found in hyperglycemia compared with normoglycemia (29 +/- 9 mm3 vs 4 +/- 4 mm3, P less than 0.001). Fifteen-minute artery occlusion induced even more damage in both hyper- and normoglycemia (63 +/- 20 mm3 vs 13 +/- 12 mm3, P less than 0.006). The lateral part of striatum was infarcted in all hyperglycemic animals exposed to 10 or 15 min of ischemia. In the same area selective neuronal injury occurred in 6 out of 9 normoglycemic animals. The findings show that hyperglycemia increases brain damage during transient ischemia by conversion of selective neuronal injury into cerebral infarction.