The changes in Na current during development were studied in the dorsal root ganglion (DRG) cells using the whole-cell patch-clamp technique. Cells obtained from rats 1-3 and 5-8 days after birth were cultured and their Na currents were compared. On top of the two types of Na currents reported in these cells (fast-FA current and slow-S current) a new fast current was found (FN). The main characteristics of the three currents are: (i) The voltages of activation are -37, -36 and -23 mV for the FN, FA and S currents, respectively. (ii) The activation and inactivation kinetics of FN and FA currents are about five times faster than those of the S current. (iii) The voltages at which inactivation reaches 50% are -139, -75 and -23 mV for the FN, FA and S currents, respectively. The kinetics and voltage-dependent parameters of the three currents and their density do not change during the first eight days after birth. However, their relative frequency in the cells changes. In the 1-3 day-old rats the percent of cells with S, FA, and mixed S + FN currents is 22, 18, and 60% of the cells, respectively. In the 5-8 day-old, the percent of cells with S, FA, and FN + S is 10, 66 and 22%. The relative increase in the frequency of cells with FA current during development can contribute to the ease of action potential generation compared with cells with FN currents, which are almost completely inactivated under physiological conditions. The predominance of FA cells also results in a significant decrease in the relative frequency of cells with the high-threshold, slow current. Antibodies directed against a part of the S4 region of internal repeat I of the sodium channel (C1+, amino acids 210-223, eel channel numbering) were found to shift the voltage dependence of FA current inactivation (but not of FN or S currents) to more negative potentials. The effect was found only when the antibodies were applied externally. The results suggest that FN, FA and S types of Na currents are generated by channels, which are different in the topography of the C1+ region in the membrane.