Several types of Na+ currents have previously been demonstrated in dorsal root ganglion (DRG) neurons isolated from neonatal rats, but their expression in adult neurons has not been studied. Na+ current properties in adult dorsal root ganglion (DRG) neurons of defined size class were investigated in isolated neurons maintained in primary culture using a combination of microelectrode current clamp, patch voltage clamp and immunocytochemical techniques. Intracellular current clamp recordings identified differing relative contributions of TTX-sensitive and -resistant inward currents to action potential waveforms in DRG neuronal populations of defined size. Patch voltage clamp recordings identified three distinct kinetic types of Na+ current differentially distributed among these size classes of DRG neurons. 'Small' DRG neurons co-express two types of Na+ current: (i) a rapidly-inactivating, TTX-sensitive 'fast' current and (ii) a slowly-activating and -inactivating, TTX-resistant 'slow' current. The TTX-sensitive Na+ current in these cells was almost completely inactivated at typical resting potentials. 'Large' cells expressed a single TTX-sensitive Na+ current identified as 'intermediate' by its inactivation rate constants. 'Medium'-sized neurons either co-expressed 'fast' and 'slow' current or expressed only 'intermediate' current. Na+ channel expression in these size classes was also measured by immunocytochemical techniques. An antibody against brain-type Na+ channels (Ab7493)10 labeled small and large neurons with similar intensity. These results demonstrate that three types of Na+ currents can be detected which correlate with electrogenic properties of physiologically and anatomically distinct populations of adult rat DRG neurons.