Cell cultures were prepared from human fetal and newborn dorsal root ganglia (DRG) and maintained in culture medium with normal (4 mM) and elevated (20 mM) potassium (K). Both types of cultures were transferred to 4 mM K, and the electrical membrane properties of the DRG neurons characterized by an extensive investigation of resting membrane potential, specific membrane resistance (Rm) and capacitance, time constant (tau), rheobasic current, and various aspects of the action potential, including overshoot, duration (DT), afterhyperpolarization and absolute refractory period (ARP). No previous quantitative determinations of these membrane properties for human neurons could be found. However, our values were in good agreement with those reported for the DRG neurons of other mammalian species both in vitro and in situ. Significant correlations occurred between membrane properties and the developmental age of each specimen used for culturing. However, the data must be interpreted cautiously due to the limited number of specimens used (4) and the possibility of differential responses of the specimens to the experimental situation. Precultivation in high K medium had profound effects on both passive and active membrane properties: the primary effect being to reduce Rm, and as a result tau, DT, and ARP were also reduced. These observations add further support to the hypothesis that high K in vitro can substitute to some extent for trophic factors normally present in situ.