Integer multiples firing (IMF), a special temporal pattern of firing, was recently observed in spontaneous discharge from injured dorsal root ganglion (DRG) neurons. To investigate the mechanism underlying IMF, the injured DRG neurons of rat were recorded intracellulary. Of 64 recorded A-neurons discharging spontaneously, eight fired spikes in the IMF pattern. Interspike interval (ISI) time series of IMF showed a structure of distinct bands on scatter map. Regular subthreshold membrane oscillations (SMOs) with relatively stable amplitude and frequency were observed on all eight IMF neurons. IMF could be induced from the neurons in periodic firing by local application of tetrodotoxin (TTX), a Na+ channel antagonist. During this process, the amplitude of SMOs varied markedly. Some SMOs were below action potential threshold so that they did not trigger spikes. Nor did some SMOs, though their amplitude were obviously beyond the threshold measured from nearby spikes. The results indicate that regular SMOs existent in injured DRG A-neurons underlie IMF, and the TTX-induced transformation of firing pattern from periodic to integer multiples may occur in two ways: decreasing the amplitude of SMOs and/or elevating action potential threshold.