Clinical, morphological, and biochemical findings reported in the spontaneously diabetic BB-Wistar rat strongly indicate that this animal may be a true model of human insulin-dependent diabetes mellitus. As such, it may provide a valuable model in which to study the neuropathic conditions of diabetes. We examined somatic peripheral nerves at five levels in a longitudinal fashion using quantitative morphological techniques. Myelinated fiber atrophy occurred earlier in sensory nerves than in motor nerves and showed a distal to proximal progression with duration of diabetes. Axon/myelin ratios revealed disproportionate shrinkage of axons evident already after 4 months of diabetes in the sural nerve and only after 8 months in the peroneal nerve. Endoneurial edema could not be demonstrated by morphometric means in diabetic nerves. We conclude that the distal symmetrical polyneuropathy in diabetes can be characterized as a mainly sensory axonopathy of dying-back type.