The temporal course of diabetic polyneuropathy in a rat model plays a critical role in studies on diabetic polyneuropathy treatment. In this study, the temporal course of neuropathic symptoms was investigated in diabetic rats induced by streptozotocin and evaluated by nerve conduction velocity and behavioral assays, including the von Frey test for mechanical allodynia and the hot plate test for hyperalgesia. The results revealed that both mechanical allodynia and heat hyperalgesia started on the 2nd week, while nerve conduction velocity significantly decreased from the 1st week. In addition, the severity of allodynia did not change after the 3rd week. Hyperalgesia and nerve conduction velocity progressively aggravated even to the 8th week. Transmission electron microscopy showed that loss of unmyelinated axons, loosening of the myelin structure, and thickening of the perineurium layer were visible from the 4th week and worsened on the 8th week. Differences in the temporal course of neuropathic symptoms are discussed.