During ontogeny retinal ganglion cells manifest pronounced changes in excitable membrane properties. To further our understanding of the ionic conductances underlying such functional changes, the whole-cell voltage-clamp variation of the patch-clamp technique was used to record potassium currents in 220 ganglion cells dissociated from cat retinas ranging in age from embryonic day 31 to postnatal day 10. Potassium currents were isolated by blocking voltage-gated Na+ and Ca2+ currents with tetrodoxin (TTX) and CoCl2 respectively and were characterized by their pharmacology, kinetics and voltage-dependence of activation and inactivation. In all cases, a combination of three currents accounted for the total outward calcium-independent K+ current: (i) a steady linear conductance; (ii) a voltage-gated transient current, lA, and (iii) a voltage-gated sustained current, lk. Both voltage-gated currents were affected by the application of 4-aminopyridine and tetraethylammonia (TEA): lA showed a greater sensitivity to 4-aminopyridine, while lk was more sensitive to TEA. Both voltagegated currents were present throughout the developmental period examined; however, the percentage of retinal ganglion cells (RGCs) expressing lA showed a marked decline from 82% at E31 to 45% at postnatal ages. During this developmental period there was an increase in the density of the two voltage-gated and the linear conductance. Additionally, with maturation, significantly slower inactivation kinetics were observed for lK. These findings, and our previous results dealing with maturational changes in the TTX-sensitive voltage-gated Na current, are related to the generation of excitability in developing retinal ganglion cells. Furthermore, the presence of cells with and without transient K+ conductance throughout development suggests that the different spiking patterns observed in RGC classes may be partially due to differences in their membrane properties.