Intracellular recording techniques were used to study the effects of apamin (APA), a selective inhibitor of one type of Ca(2+)-activated K+ channel, on the electroresponsive properties of dopamine (DA)-containing neurons within the zona compacta of the substantia nigra (SNc) in rat. Bath application of APA (1 microM) blocked the slow component of a complex post-spike afterhyperpolarization (AHPs) without affecting other characteristics of the action potential. Blockade of AHPs was accompanied by an increase in the number and frequency of action potentials evoked by depolarizing current pulses. However, APA failed to affect the cellular mechanisms underlying spike frequency adaptation or post-stimulus inhibitory period. These data indicate that AHPs can exert a strong influence on the interspike interval but is probably not involved in regulating slower adaptive neuronal responses.