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 an EC50 of 8 microM. Likewise, dendrotoxin partially blocked both currents but with an EC50 of 21 nM. In the absence of Ca channel blockers, a prominent Ca-dependent K+ current was observed. The kinetics of whole-cell potassium currents varied widely among cells, perhaps reflecting the different functional properties of sensory neurons. We also investigated the effects of elevating intracellular cyclic AMP and applying opioids on K+ currents. Membrane-permanent analogs of cyclic AMP and phosphodiesterase inhibitors caused small reductions in voltage-dependent outward current. In contrast, forskolin produced a large reduction in outward current. This response was not solely mediated by cyclic AMP, since large responses were elicited with an inactive congener, 1,9-dideoxyforskolin, but not with the active, water-soluble congener, 7-deacetyl-6-[N-acetylglycyl]-forskolin. Surprisingly, opioids had no effect on resting or voltage-dependent K+ conductances. However, opioid inhibition of Ca2+ currents and Ca-dependent K+ currents was observed. The failure to demonstrate opioid modulation of resting or voltage dependent K+ currents suggests that modulation of Ca2+ currents is the principal mechanism for the inhibitory effect of opioids on sensory neurons.