SUMMARY1. We studied the effect of locally applied 2-amino-4-phosphonobutyric acid (APB) on the response of X-type cat retinal ganglion cells to stimulation by sinusoidal gratings of photopic mean luminance.2. Application of APB produced a decrease in the mean firing rate of on-centre X cells, but an increase in the mean firing rate of off-centre X cells.3. The response and responsivity of both on-and off-centre X cells to drifting sinusoidal gratings were suppressed by APB. Under control conditions, the response amplitude was linear with grating contrast for contrasts less than 10 %. During APB application, the slope of the contrast response curve was reduced, and linearity was maintained for a larger range of contrast.4. Assuming that APB acts selectively on the cone to depolarizing bipolar cell synapse, the suppression of the response of on-and off-centre X cells suggests a functional link between depolarizing bipolar cells and both types of X cell.5. Because APB reduces the centre response of both on-and off-centre X cells, caution should be observed in interpreting the effect of APB on higher visual centres in terms of the blockage of 'on' signals leaving the retina.6. By analysing the spatial frequency tuning curve before and during APB application in terms of the difference-of-Gaussians model of receptive field structure, it was found that the balance between the integrated strengths of the centre and surround was not modified by APB. APB, however, had a differential effect on the summating area of the centre and surround. While the centre radius was unaffected by APB, the surround radius was reduced. This suggests that the peak sensitivity of the surround mechanism was reduced less severely than the peak sensitivity of the centre mechanism.7. Strychnine also suppressed the response of the centre mechanism of both onand off-centre X cells to drifting gratings.