The spontaneously diabetic "BB" Wistar rat was examined for evidence of peripheral nerve abnormalities by a combined morphologic and physiologic approach. The studies were done on rats kept severely hyperglycemic and frequently ketotic. The peripheral nerves of the lower extremities, including the most distal nerves of the intrinsic foot muscles, revealed only minimal abnormalities by histologic and morphometric examination. Sciatic nerve conduction studies, however, measured over a 2-month period did not show a significant slowing in diabetics compared with age-matched controls, as well as between diabetics and weight-matched controls. In addition, fast anterograde axoplasmic transport studies were correlated with serum glucose results. Rats maintained severely hyperglycemic with or without ketosis had abnormal down flow rats of [3H]leucine compared to controls. Diabetic rats maintained with normal blood glucose levels showed no change in transport rates. These results suggested that persistent hyperglycemia in the "BB" Wistar rat produces significant physiologic but not significant structural abnormalities in the peripheral nerve.