In order to analyze mechanisms of directional selectivity in retinal ganglion cells, synaptic potentials were recorded intracellularly in the eye-cup preparation of the bullfrog. Some preparations were perfused with tetrodotoxin to isolate the synaptic potentials from spike discharges. The cells were classified into ON-OFF-, ON-, and OFF-type by the responses to a stationary flashing light. About 40% of the cells in each type showed directional selectivity to a moving stimulus across their receptive fields. ON-OFF-type cells produced both EPSP and IPSP, and their amplitudes varied depending on the direction of stimulus movement. These findings suggest that the directional selectivity in ON-OFF-type cells could be attributed to the balance of counter-acting postsynaptic potentials. On the other hand, ON- and OFF-type cells responded to the moving stimuli only with EPSP's, and no IPSP was detected. An EPSP evoked by the preferred movement of stimulus was larger in amplitude than that produced by the null movement, thus resulting in directional selectivity. The neural mechanism responsible for the difference in EPSP amplitude in ON- and OFF-type cells probably lies presynaptic to the ganglion cells.