The development of precise retinofugal projections is dependent on activity-mediated events, but as yet nothing is known about the ontogeny of excitable membrane properties in retinal ganglion cells (RGCs). In order to begin to understand how functional maturity is attained in these neurons, whole-cell patch-clamp recordings were obtained from acutely dissociated RGCs of fetal and postnatal timed- pregnant cats. Current-clamp recordings revealed a pronounced developmental increase in the proportion of RGCs capable of generating action potentials. At embryonic day 30 (E30), 5 weeks before birth and during a time when RGCs are still being generated, electrical stimulation elicited spikes in only a third of the cells. None of these neurons were capable of multiple discharges in response to maintained depolarization. The proportion of spiking neurons increased during ontogeny, such that by E55 all RGCs could be induced to generate action potentials, with the majority manifesting repetitive spiking patterns. Application of tetrodotoxin abolished spike activity of all fetal RGCs, indicating that sodium-mediated action potentials are present very early in development. At the same time, voltage-clamp recordings revealed significant ontogenetic modifications in several key properties of the sodium currents (INa). These were (1) a twofold increase in Na current densities; (2) a shift in the voltage dependence of both activation and steady state inactivation: with maturity, sodium currents activate at more negative potentials, while steady state inactivation of INa occurs at less negative potentials; and (3) a decrease in decay time constants of the Na current, at membrane potentials negative to -15 mV. These developmental changes were largely restricted to the period of axon ingrowth (E30-E38), suggesting that maturation of INa is not the limiting factor for the onset of activity- dependent restructuring of retinofugal projections.