In this paper we have examined the ultrastructural changes in myelinated fibre structure after the administration of streptozotocin to Sprague-Dawley rats which had passed the rapid growth period. Myelinated fibre size in the tibial nerve was found to be less in diabetic animals 4 and 6 months after the induction of diabetes, when compared to age-matched controls, but not less than onset. The relative contributions of axon and myelin to this reduction in fibre dimensions were examined. When myelin area was plotted against axon area (derived from perimeter) it showed that the pathological insult of diabetes had a greater effect on the rate of myelin production. The incidence of axonal glycogenosomes was also assessed. These results are discussed in detail.