A retinal ganglion cell receptive field comprises an excitatory center and an inhibitory surround. The surround has two components: one driven by horizontal cells at the first synaptic layer, and one driven by amacrine cells at the second synaptic layer. Here, we characterized how amacrine cells inhibit the center response of ON- and OFF-center Y-type ganglion cells in the in vitro guinea pig retina. A high spatial frequency grating (4-5 cyc mm-1), beyond the spatial resolution of horizontal cells, drifted in the ganglion cell receptive field periphery to stimulate amacrine cells. The peripheral grating suppressed the ganglion cell spiking response to a central spot. Suppression of spiking was strongest and observed most consistently in OFF cells. In intracellular recordings, the grating suppressed the subthreshold membrane potential in two ways: a reduced slope (gain) of the stimulus-response curve by ~20-30% and, in OFF cells, a tonic ~1 mV hyperpolarization. In voltage-clamp, the grating increased an inhibitory conductance in all cells and simultaneously decreased an excitatory conductance in OFF cells. To determine if center response inhibition was presynaptic or postsynaptic (shunting), we measured center response gain under voltage-clamp and current-clamp conditions. Under both conditions, the peripheral grating reduced center response gain similarly. This result suggests that reduced gain in the ganglion cell subthreshold center response reflects inhibition of presynaptic bipolar terminals. Thus, amacrine cells suppressed ganglion cell center response gain primarily by inhibiting bipolar cell glutamate release.