Retinal degenerate C3H mice were tested in the open field under 2 illumination conditions before, during, and after the ages at which the early degenerate process occurs to determine whether the progressive destruction of the retina would result in the loss of brightness discrimination ability. From data collected on mice tested at 10, 14, 18, 22, 26, and 30 days of age, longitudinal and cross-sectional analyses were performed upon activity and and defecation scores to separate the effects of prior test experience and age at testing. Whereas illumination differences had no effect upon defecation scores, activity differences under the 2 illumination levels formed an inverted U-shaped function, with the greatest activity difference at 22 days of age. Prior experience was found to have no effect upon total activity scores at a particular age, although it did affect the pattern of activity over test days at the later ages. Naive animals tended to defecate less on their first day than did animals of the same age with prior test experience. The results of this experiment provide further evidence that retinal degeneration does not lead to complete blindness in this mouse strain, although it does lead to a loss of brightness discrimination ability following the degeneration period.Mice afflicted with retinal degeneration (rd), an inherited abnormality, undergo spontaneous degeneration of the retina during the second postnatal week and appear to lose their visual capacities thereafter. Anatomical evidence reported by Tansley