Using tight-seal recordings from rat spinal cord slices, intracellular labelling and computer simulation we analysed the mechanisms of spike frequency adaptation in substantia gelatinosa (SG) neurons. Adapting-firing neurons (AFNs) generated short bursts of spikes during sustained depolarisation and were mostly found in lateral SG. The firing pattern and the shape of single spikes did not change after substitution of Ca²⁺ with Co²⁺, Mg²⁺ or Cd²⁺ indicating that Ca²⁺-dependent conductances do not contribute to adapting firing. Transient K_Acurrent was small and completely inactivated at resting potential suggesting that adapting firing was mainly generated by voltage-gated Na⁺ and delayed-rectifier K⁺(K_DR)currents. Although these currents were similar to those previously described in tonic-firing neurons (TFNs), we found that Na⁺ and K_DR currents were smaller in AFNs. Discharge pattern in TFNs could be reversibly converted into that typical of AFNs in the presence of tetrodotoxin but not tetraethylammonium, suggesting that lower Na⁺ conductance is more critical for appearance of firing adaptation. Intracellularly labelled AFNs showed specific morphological features and preserved long extensively branching axons, indicating that smaller Na⁺ conductance could not result from the axon cut. Computer simulation has further revealed that down-regulation of Na⁺ conductance represents an effective mechanism for induction of firing adaptation. It is suggested that the cell-specific regulation of Na⁺ channel expression can be an important factor underlying diversity of firing patterns in SG neurons.