The effect of 2 months aminoguanidine treatment on nerve conduction abnormalities was studied in streptozotocin-diabetic rats. Treatment with aminoguanidine from the induction of diabetes mellitus prevented a 22% decrease in sciatic motor nerve conduction velocity (p less than 0.001), and a 10% deficit in sensory saphenous conduction velocity (p less than 0.01). There was a 49% increase in resistance of sciatic nerve to hypoxic conduction failure in vitro. This was not significantly affected by aminoguanidine treatment. Sciatic nerve polyol pathway metabolites, sorbitol and fructose, were elevated 10-fold by diabetes (p less than 0.001). Myo-inositol levels were 18% decreased by diabetes. Aminoguanidine treatment had no significant effect on sorbitol, fructose or myo-inositol levels. Aminoguanidine has been identified as both an inhibitor of the formation of advanced glycation end products, and an aldose reductase inhibitor. The data suggest that beneficial actions on nerve function do not depend on the latter property. They support the notion that advanced glycation end products contribute to the aetiology of early diabetic neuropathy, possibly acting via a vascular mechanism, and that aminoguanidine treatment may have therapeutic applications.