OBJECTIVE To examine the functional, metabolic and structural abnormalities in peripheral nerve in the spontaneously Type II (non-insulin-dependent) diabetic BBZDR/Wor rat and compare these data with those in the Type I (insulin-dependent) diabetic BB/Wor rat. METHODS Animals were examined at 6 and 14 months of diabetes. Nerve conduction velocity was measured longitudinally. Nerve polyols were analysed using gas liquid chromatography and Na/K(+)-ATPase activity was measured enzymatically. Light and electron microscopic techniques were used for nerve morphometry. RESULTS Diabetic BBZDR/Wor rats showed a slowly progressive nerve conduction defect that reached 17% (p < 0.01) at 14 months. There was a decrease in Na+/K(+)-ATPase of 35% (p < 0.05). Structurally, there were mild myelinated fibre atrophy (p < 0.05), mild or absent changes of the node of Ranvier, but significant (p < 0.001) segmental demyelination and Wallerian degeneration. These findings point to a more severe nerve conduction defect, severe myelinated fibre atrophy and profound nodal changes in Type I spontaneously diabetic BB/Wor rats maintained at the same hyperglycaemic concentrations. CONCLUSIONS We conclude that other factors, beside hyperglycaemia, are involved in the pathogenesis of the more severe Type I diabetic neuropathy which possibly involve insulin and C-peptide deficiencies.