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Using the whole-cell patch-clamp technique on acutely dissociated and cultured adult rat sensory neurons, we characterized the K⁺ currents by voltage dependence, kinetics, calcium dependence, and pharmacology. In the presence of Ca channel blockers, the cells heterogeneously expressed transient and sustained outward K⁺ currents. The transient current was a high-threshold A-current which activated at potentials greater than −30 mV and was blocked by 4-aminopyridine. Some of the sustained current was classified as a delayed rectifier. It demonstrated shallow voltage-dependent inactivation and was blocked by tetraethylammonium. Capsaicin produced large reductions in both transient and sustained currents with anec₅₀of 8 μM. Likewise, dendrotoxin partially blocked both currents but with anec₅₀ of 21 nM. In the absence of Ca channel blockers, a prominent Ca-dependent K⁺ current was observed. The …