The influence of nerve growth factor (NGF) on Na currents of rat dorsal root ganglia (DRG) was studied in neurons obtained from newborns and cultured for 2-30 hr in serum-free defined medium (SFM). Cell survival for the period studied was 78-87% both with and without NGF. Na currents were detected in all cells cultured for 6-9 hr. They were also detected after 2 hr in culture in 21.5% of the cells cultured without NGF (-NGF cells), and in 91.5% of the cells cultured with NGF (+NGF cells). Current density of the -NGF cells was 2.3 and 2 pA/microns 2 after growth for 2 and 6-9 hr, respectively, compared to 3.0 and 3.9 pA/microns 2 for the +NGF cells. The +NGF cells were separated into fast (F), Intermediate (I) and slow (S) cells, based on the Na current they expressed, while -NGF cells were all of the I type. F, I and S currents differed in their voltage-dependent inactivation (Vh50 = -79, -28 and -20 mV), kinetics of inactivation (tau h = 0.55, 1.3 and 7.75 msec), and TTX sensitivity (Ki = 60, 550 and 1100 nM). All currents were depressed by [Ca]0 with a KdCa of 22, 17 and 8 mM for F, I and S currents, respectively. Current density of F and S currents was 5.5 and 5 pA/micron 2 for the I current. The concentration-dependent curve of I current vs. TTX indicated that I current has two sites: one with F-like and another with S-like Ki for TTX. Hybridization of F and S currents yielded I-like currents. Thus, the major effect of NGF on Na currents in SFM is the acceleration of Na current acquisition and diversity, reflected in an increase of either the S or F type in a cell.