Nerve regeneration across a 10 mm gap through an implanted silicone tube was delayed in galactose-fed rats two and four weeks after transecting the nerve. This experimental metabolic neuropathy resembles diabetic neuropathy in which nerve regeneration is also delayed. Experiments were performed by introducing opposite ends of divided sciatic nerves into close-fitting silicone tubes, leaving a 10 mm gap. Growth of neurites across this gap was monitored by electron microscopy performed in sections at regular intervals of 2 mm from proximal to distal stumps. After two weeks some difference was apparent; axons advanced 1.4 +/- 0.4 mm in galactose-fed rats versus 3.5 +/- 1.5 mm in controls. Myelination did not progress beyond 1 mm in galactose-fed rats. Differences were greater between the two groups at four weeks. The growth of axons in galactose rats was 3.5 +/- 0.2 mm versus 9.4 +/- 0.1 mm in control nerves. In addition the size of the regenerating stump was much greater in control rats. Qualitative differences were also noticed during electron microscopic comparison of control and galactose-treated rats. The dystrophic axons seen in treated rats had abnormal electron-dense organelles, lamellated bodies, vesicles and tubular structures, as well as numerous glycogen granules. Abnormalities of spatial orientation were also noted. Unlike control axons which grew parallel with the long axis of the tube, regenerating axons in experimental animals were seen deviating from the axis at 90 degrees angles. Both immature sprouts and myelinating axons showed abnormal plasticity. Ultrastructural differences were also noted in Schwann cells, macrophages and vessels.