The striatum is considered to be critical for the control of goal-directed action, with the lateral dorsal striatum (latDS) being implicated in modulation of habits and the nucleus accumbens thought to represent a limbic–motor interface. Although medium spiny neurons from different striatal subregions exhibit many similar properties, differential firing and synaptic plasticity could contribute to the varied behavioral roles across subregions. Here, we examined the contribution of small-conductance calcium-activated potassium channels (SKs) to action potential generation and synaptic plasticity in adult rat latDS and nucleus accumbens shell (NAS) projection neurons in vitro. The SK-selective antagonist apamin exerted a prominent effect on latDS firing, significantly decreasing the interspike interval. Furthermore, prolonged latDS depolarization increased the interspike interval and reduced firing, and this enhancement was reversed by apamin. In contrast, NAS neurons exhibited greater basal firing rates and less regulation of firing by SK inhibition and prolonged depolarization. LatDS neurons also had greater SK currents than NAS neurons under voltage-clamp. Importantly, SK inhibition with apamin facilitated long-term depression (LTD) induction in the latDS but not the NAS, without alterations in glutamate release. In addition, SK activation in the latDS prevented LTD induction. Greater SK function in the latDS than in the NAS was not secondary to differences in sodium or inwardly rectifying potassium channel function, and apamin enhancement of firing did not reflect indirect action through cholinergic interneurons. Thus, these data demonstrate that SKs are potent modulators of action potential generation and LTD in the dorsal striatum, and could represent a fundamental cellular mechanism through which habits are regulated.