The hypothesis that nerves in diabetes mellitus exhibit an increased susceptibility to compression was experimentally tested. Inhibition of fast axonal transport was induced by local compression in sciatic nerves of rats with streptozotocin-induced diabetes mellitus. Fast anterograde axonal transport was measured after application of 3H-leucine to the motor neurone cell bodies in the spinal cord. The sciatic nerve was subjected to local, graded compression in vivo by a small compression chamber. The amount of accumulation of proteins was quantified by calculation of a transport block ratio. Compression at 30 mm Hg for 3 h induced a significantly greater (p less than 0.05) accumulation of axonally transported proteins at the site of compression in nerves of diabetic animals (transport block ratio: 1.01 +/- 0.35; n = 7) than in nerves of controls (0.67 +/- 0.16; n = 7). Accumulation was significantly higher in ligature experiments of both control (1.34 +/- 0.44; n = 8; p less than 0.01) and diabetic animals (1.45 +/- 0.30; n = 8; p less than 0.05), indicating that the block of transport in compressed nerves was incomplete. Neither sham compressed diabetic (0.50 +/- 0.09; n = 6) nor control (0.49 +/- 0.11; n = 6) nerves showed any block of axonal transport. The possible causes of the increased inhibition of fast axonal transport in diabetic rats are discussed. The results indicate that diabetes may lead to an increased susceptibility of peripheral nerves to compression.