Psychophysical techniques have been employed to study the visual response to sinusoidally varying spatial stimuli. The measurements were performed for four different average luminances of the sine-wave distribution (0·24, 0·59, 1·31 and 2·00 cd/m2) and over the contrast region 0·05-0·70. The spatial frequency used was 1·74 lines/mm on the retina. Various non-linear properties of the eye in the mesopic region were elucidated, and a possible explanation proposed in terms of inhibitory and excitatory phenomena. Evidence has therefore been presented opposing the general application of linear filter theory to the visual system.