In vertebrate retinae, APB (2-amino-4-phosphonobutyrate) selectively activates glutamate receptors of ON-bipolar cells. Thus, APB has been used as a pharmacological tool for studying the functional retinal circuitry and the inputs to higher-order pathways of the visual centres. Direct action of APB on ganglion cells has not previously been demonstrated. We now report, using whole-cell patch-clamp recordings, that APB has a direct inhibitory effect on ganglion cells isolated from newt retina. APB induced currents were dependent upon the intracellular Cl− concentration. Their reversal potential values agreed with theoretical values of Cl− equilibrium potential. Strychnine, a glycine receptor blocker, inhibited the APB response. These results may suggest that APB activates the Cl− channels of ganglion cells through glycine receptors.