THE modification of critical flicker fusion frequency by intermittent light of varying frequency is unchanged by such physiological procedures as hyperventilation and rebreathing1, and by drugs2,3, which are known to alter mean critical flicker fusion frequency. Wave-length and the light–dark ratio of the light source4 are important factors in determining binocular critical flicker fusion frequency, and the state of luminance of the opposite eye in determining monocular critical flicker fusion frequency5. Their possible influence on the adaptation phenomenon has, therefore, been investigated.