This study examined the effect of streptozotocin diabetes of 5 weeks duration on the profile of slow orthogradely transported radiolabelled protein in rat sciatic motoneurones. The diabetic rats showed a retardation of the tail of the slow-component profile. This selective retardation was unaffected by treatment with an aldose reductase inhibitor, although this treatment reduced the accumulation of sorbitol and prevented the depletion of myo-inositol in the sciatic nerves of the treated diabetic rats. Other groups, treated with myo-inositol, had normal or elevated sciatic nerve myo-inositol levels in the presence of accumulated sorbitol. The axonal transport profiles from both control and diabetic myoinositol-treated groups gave normal tail velocities but an altered shape such that retardation of the tail of the profile may have been present in both. The study concludes that rats with 5 weeks streptozotocin diabetes show retardation of the velocity of the most slowly transported proteins in sciatic motoneurones, and that this defect is not linked to the polyol pathway.