Phrenic neuropathies are increasingly recognized in peripheral neuropathies but reports on experimental models of the phrenic nerves diabetic neuropathy are scanty. In the present study, we investigated the phrenic nerve neuropathy, due to experimental diabetes induced by streptozotocin (STZ) and the evolution of this neuropathy in diabetic rats treated with insulin. Proximal and distal segments of the left and right phrenic nerves were morphologically and morphometrically evaluated, from rats rendered diabetic for 12 weeks, by injection of STZ. Control rats received vehicle. Treated rats received a single subcutaneous injection of insulin on a daily basis. The nerves were prepared for light microcopy study by means of conventional techniques. Morphometry was carried out with the aid of computer software. The phrenic nerves of diabetic rats showed smaller myelinated axon diameters compared to controls. The g ratio was significantly smaller for myelinated fibers from diabetic rats compared to controls. Insulin treatment prevented these alterations. Histograms of size distribution for myelinated fibers and axons from control rats were bimodal. For diabetic animals, the myelinated fiber histogram was bimodal while the axon distribution turned to be unimodal. Insulin treatment also prevented these alterations. Our results confirm the phrenic nerve neuropathy in this experimental model of diabetes and suggest that conventional insulin treatment was able to prevent and/or correct the myelinated axon commitment by diabetes.