Spontaneous, retinal degeneration in the WBN/Kob rat, a new diabetic strain, was examined by behavior pattern, electroretinogram (ERG) and histopathology. Histopathological evidence of retinal degeneration was first detected at about 30 weeks of age in male and female rats. The earliest change in the retina was loss of rads and cones, and the inner layer was gradually involved. The retina showed marked atrophy in the thinned inner plexiform layer and a decreased number of ganglionic cells by 90 weeks. Severe retinal change was observed in almost all rats over 90 weeks old. Prior to the onset of histological change, the ERG showed prominent change. Amplitude of a- and b-waves began to be reduced at 17 weeks and was non-recordable at 32 weeks. This retinal abnormality was thought to be hereditary, but further investigation is needed to clarify the relationship between the genetic background of this change and diabetes.