Using the whole-cell patch-clamp recording technique, resting membrane potentials (RPs), action potential (AP) waveforms, and the properties of voltage-activated inward Na+ (lNa) and Ca2+ (lCa) currents and outward K+ (lA,lK) currents were examined in embryonic and neonatal rat superior cervical ganglion (SCG) neurons as a function of time during development in vivo and in vitro. The passive and active membrane properties of neonatal SCG cells examined less than or equal to 24 hr after isolation were similar to those described previously for adult SCG neurons and for neonatal SCG cells maintained for several weeks in culture. Since recordings were obtained within hours of cell dissociations, it is assumed that the results reflect the membrane properties of neonatal SCG neurons in vivo at the time of isolation. When neonatal cells were examined as a function of time (up to approximately 2-3 weeks) in vitro, neither RPs nor AP waveforms varied measurably. Although absolute (inward and outward) current amplitudes increased in cells maintained in vitro, in parallel with increases in cell size, no changes in the time- or voltage-dependent properties of the currents were observed. Similar results were obtained for cells isolated on or after embryonic day 18.5 (greater than or equal to E 18.5). The membrane properties of E 14.5-16.5 SCG cells examined less than or equal to 24 hr after isolation, in contrast, were significantly different: mean lCa density was higher, and APs were broader than in greater than or equal to E 18.5 cells, and, in addition, lA was absent in these cells. When E 14.5-16.5 cells were examined after approximately 1 week in vitro, lCa densities and AP waveforms were indistinguishable from those in greater than or equal to E 18.5 SCG neurons, and lA was present. These studies reveal that rat SCG neurons are electrophysiologically mature early in development. Even lA, which seems to be the last voltage-gated current to develop, appears well before birth. As the appearance of lA correlates with decreased membrane excitability and AP shortening, it seems likely that the development of lA either reflects or regulates a marked change in the overall input and output properties of developing sympathetic neurons.