Conduction velocity of rat sciatic nerve, estimated both in vivo and in vitro, did not change during 2--21 weeks following induction of diabetes by injection of streptozotocin. During this period blood sugars were in the range 17.6--30.4 mM and symptoms of uncontrolled diabetes were noted. The failure of velocity to decrease contrasts strongly with several previous reports, while the failure to increase with age may be explained with the aid of published morphometric data. Experiments on nerve fibers isolated from streptozotocin-diabetic rats rats confirmed that the nerve function was normal in respect of action and resting potential. Potential clamp analysis of nodal membrane similarly indicated that these fibres had normal permeability properties.