The spontaneously diabetic BB/W-rat has emerged as an important model system for somatic and autonomic diabetic polyneuropathy. In this study we examined visual evoked potentials and the presence of morphometric and structural changes in the optic nerve and the retinal ganglion cells and their afferent axons contained in the retinal nerve fibre layer. A six-month duration of diabetes mellitus was associated with significant increases in the latencies of the visual evoked potentials. The latency of the first positive potential showed a 44% increase, and that of the first negative potential was prolonged by 41%. No significant changes were demonstrated at any of the amplitudes. In the optic nerve mean myelinated fibre size was significantly reduced to 82% of control values, which was accounted for by a significant reduction in axonal size. Axo-glial dysjunction, a prominent structural defect of diabetic somato-sensory neuropathy in both man and diabetic rodents, was non-significantly increased in the optic nerve. In diabetic animals retinal ganglion cells displayed dystrophic changes. No such changes were observed in age- and sex-matched control animals. Proximal axons of the retinal nerve fibre layer showed an increase in dystrophic axons in diabetic BB/W-rats. Morphometric analysis of optic nerve capillaries revealed no abnormalities except for basement membrane thickening. The present data suggest that the diabetic BB/W-rat develops a central sensory neuropathy, characterized functionally by prolonged latencies of the visual evoked potentials and structurally by an axonopathy of optic nerve fibres.