Extrafoveal thresholds for 660 nm test flashes were measured for the two eyes separately following 30 min of dark adaptation. Then, the right eye was exposed to red light while the left eye was covered. Thresholds in the adapted right eye recovered to a plateau raised about 0.3 log units above the level measured in the dark. In contrast, thresholds in the nonlight-adapted left eye dropped by about 0.3 log units. These effects persist for about 15 min. They may be accounted for in terms of a hypothetical interocular inhibitory mechanism. Results of changing the intensity, duration and wavelength of the light adaptation upon this effect are described.