The thresholds of dark adapted RCS rats and controls were estimated from optokinetic responses to blue stimuli, and compared to thresholds derived from b-wave stimulus/response functions. The rats were tested at ages 25, 30 and 35 days, a period during which the RCS degeneration is evolving and normal retinal development is proceeding. At all ages the OKN thresholds of control rats were lower than those of the RCS rats, but only by age 30 days did b-wave thresholds discriminate RCS from controls. As the RCS disease progressed b-wave thresholds increased, but OKN thresholds did not change significantly. The discrepancy between the OKN and b-wave results may be due largely to the smaller OKN stimulus field, which probably stimulates mainly the posterior retina where the RCS degeneration is most advanced.