OBJECTIVE Psychophysical studies have shown that a dark-adapted eye exerts a tonic interocular suppression (TIS) upon spatial vision mediated by the contralateral eye. The present study was designed to demonstrate TIS by means of visual evoked potential (VEP) procedures. METHODS Evoked cortical potentials were obtained in response to reversing checkerboard patterns with fundamental Fourier frequencies between 3 and 12 cycles per degree. Responses were obtained under monocular viewing conditions when the contralateral "adapting" eye was dark adapted, under monocular viewing conditions when the adapted state of the adapting eye was experimentally manipulated, or under binocular viewing conditions. Data were collected from three healthy young men, two native regarding purpose of experimentation. RESULTS Regardless of spatial frequency, monocular responses evoked by stimulating a "test eye" were always smaller in amplitude when the contralateral adapting eye was dark adapted than when adapted to a dim, homogeneous field. The monocular evoked response obtained in the presence of an interocular adapting field was similar in amplitude to the binocular evoked response. During dark adaptation of the contralateral adapting eye, the amplitude of the monocular evoked response decreased: the time course of this decline follows that of psychophysically measured rod thresholds in the directly adapted eye. CONCLUSIONS TIS is easily demonstrated by means of VEP as well as psychophysical procedures. The well-known increase in VEP amplitude resulting from binocular viewing may be attributable to the removal of TIS rather than to "physiologic, binocular summation."