In 33 patients of symmetrical diabetic polyneuropathy, blink reflexes and direct responses were measured and compared with those of 20 normal subjects. They were also compared with periphearal nerve conduction velocities, duration of diabetes mellitus and sensory symptoms. The results were; 1. R1 latencies of blink reflex and direct response latencies were abnormally delayed in 55%, 50% respectively, and R/D ratios were within normal range. 2. R1 latencies had significant correlation with motor and mixed nerve conduction velocities but not with sensory nerve conduction velocities. 3. R1 latencies were not significantly correlated with duration of diabetes mellitus, but were well correlated with that of sensory symptoms. These findings indicate that a significant portion (more than 50%) of diabetic polyneuropathy has electrophysiological dysfunction in cranial nerves including facial and trigeminal nerves, which probably are diffusely damaged. Although it is unclear whether R1 latencies have any correlation with the severity of diabetes mellitus, it is likely that they reliably reflect the severity of neuropathy.