1. We studied the effects of 2-amino-4-phosphonobutyric acid (APB) on the response properties of rods, horizontal cells and bipolar cells in the isolated, perfused retina of the tiger salamander, Ambystoma tigrinum. A concentration of 100 microM was found to be sufficient to elicit maximal effects. 2. Rods hyperpolarized slightly upon exposure to 100 microM-APB and their response amplitudes were slightly reduced. The amplitude of the cone-generated component of the rod's response to 700 nm light was not significantly affected by APB. 3. Horizontal cells hyperpolarized by 2-5 mV upon exposure to 100 microM-APB. The rod-driven component of the horizontal cell response increased in amplitude while the cone-driven component decreased in amplitude. APB thus causes an increase in voltage gain between rods and horizontal cells and a decrease in cone/horizontal cell gain. These findings can be explained in terms of an APB-induced reduction in transmitter release from the cones. 4. APB at a concentration of 100 microM caused an increase in the length constant of the horizontal cell syncytium. Our analysis shows this to be due primarily to a 50% reduction in the coupling impedance between the cells of the syncytium. 5. The effects of APB on off-centre bipolar cells were qualitatively similar to those on horizontal cells. APB increased the amplitudes of rod-driven responses and reduced those of cone-driven responses. The length constants, both of the receptive field centre and of the surround, were increased and the strength of the surround relative to the centre was reduced by about 20%. 6. APB abolished the depolarizing light responses of the receptive field centres of on-centre bipolar cells. A hyperpolarizing response remained whose spatial properties were similar to those of the receptive field surround. We believe this response to reflect a direct (feedforward) input to on-centre bipolar cells from horizontal cells.