Effects of strong membrane depolarization on the excitability of regular spiking (RS) non-pyramidal cells in layer VI of the cat motor cortex were examined in in vitro slice preparations. RS non-pyramidal cells that expressed depolarizing afterpotential (DAP) following spikes displayed either of the two distinct responses: immediate and successive potentiation of DAP in association with repetition of firing, or slow enhancement of DAP after repetitive injections of strong depolarizing current pulses leading to burst firing. In those neurones that displayed the immediate potentiation of DAP, slow potentiation was never induced. This difference in changes of DAP was consistent with morphological differences. DAP-lacking RS non-pyramidal cells displayed neither immediate nor slow changes in excitability.