We studied the effect of local ionophoretic application of bicuculline on the response of cat lateral geniculate nucleus (laminae A) cells to stimulation by sinusoidal gratings and spots of light. Application of bicuculline produced an increase both of spontaneous and visually driven discharge of both X and Y cells. On stimulation by drifting sinusoidal gratings, the average discharge of both X and Y cells remained constant with increasing contrast under normal conditions. Application of bicuculline caused the average discharge to increase with contrast, indicating that the constancy of the average discharge was maintained by gamma-aminobutyric acid mediated inhibition. Under normal conditions, the amplitude of response modulation of both X and Y cells to sinusoidal grating stimulation increased monotonically with stimulus contrast. During bicuculline application, the slope of the contrast-response curve for X cells but not for Y cells increased, indicating that the inhibition which dampened the modulation of X cells (but not Y cells) was contrast dependent. Application of acetylcholine also increased the average discharge and the amplitude of modulation of the cell responses, but this increase did not depend on stimulus contrast. Under normal conditions, X but not Y cells showed an attenuation of response and an increase in contrast threshold to low spatial frequencies. This attenuation vanished during bicuculline application. The shape of Y-cell response curves was unaffected by bicuculline. Bicuculline had the same effect on the non-linear component of Y-cell response as on the linear component. Although bicuculline had a different effect on the response of X and Y cells to stimulation by gratings, it reduced the antagonistic surround of both X and Y cells to a similar extent (revealed by plotting the cell receptive fields with flashed spots of light).