Aims/hypothesis. Insulin and C-peptide exert neuroprotective effects and are deficient in Type I (insulin-dependent) diabetes mellitus but not in Type II (non-insulin-dependent) diabetes mellitus. These studies were designed to test the preventive and interventional effects of C-peptide replacement on diabetic polyneuropathy in the Type I diabetic BB/Wor rat. Methods. Diabetic BB/Wor rats were replaced with rat C-peptide from onset of diabetes and between 5 and 8 months of diabetes. They were examined at 2 and 8 months and compared to non-C-peptide replaced BB/Wor rats, Type II diabetic (non-C-peptide deficient) BB/Z rats and non-diabetic control rats. Animals were monitored as to hyperglycaemia and nerve conduction velocity (NCV). Acute changes such as neural Na⁺/K⁺-ATPase and paranodal swelling were examined at 2 months, morphometric and teased fiber analyses were done at 8 months. Results. C-peptide replacement for 2 months in Type I diabetic rats prevented the acute NCV defect by 59 % (p < 0.005), the neural Na⁺/K⁺-ATPase defect by 55 % (p < 0.001) and acute paranodal swelling by 61 % (p < 0.001). Eight months of C-peptide replacement prevented the chronic nerve conduction defect by 71 % (p < 0.001) and totally prevented axoglial dysjunction (p < 0.001) and paranodal demyelination (p < 0.001). C-peptide treatment from 5 to 8 months showed a 13 % (p < 0.05) improvement in NCV, a 33 % (p < 0.05) improvement in axoglial dysjunction, normalization (p < 0.001) of paranodal demyelination, repair of axonal degeneration (p < 0.01), and a fourfold (p < 0.001) increase in nerve fibre regeneration. Conclusion/interpretation. C-peptide replacement of Type I BB/Wor-rats partially prevents acute and chronic metabolic, functional and structural changes that separate Type I diabetic polyneuropathy from its Type II counterpart suggesting that C-peptide deficiency plays a pathogenetic role in Type I diabetic polyneuropathy. [Diabetologia (2001) 44: 889–897]