Dorsal root ganglion (DRG) neurons express mRNAs for many two-pore domain K(+) (K(2P)) channels that behave as background K(+) channels. To identify functional background K(+) channels in DRG neurons, we examined the properties of single-channel openings from cell-attached and inside-out patches from the cell bodies of DRG neurons. We found seven types of K(+) channels, with single-channel conductance ranging from 14 to 120 pS in 150 mM KCl bath solution. Four of these K(+) channels showed biophysical and pharmacological properties similar to TRESK (14 pS), TREK-1 (112 pS), TREK-2 (50 pS), and TRAAK (73 pS), which are members of the K(2P) channel family. The molecular identity of the three other K(+) channels could not be determined, as they showed low channel activity and were observed infrequently. Of the four K(2P) channels, the TRESK-like (14 pS) K(+) channel was most active at 24 degrees C. At 37 degrees C, the 50-pS (TREK-2 like) channel was the most active and contributed the most (69%) to the resting K(+) current, followed by the TRESK-like 14-pS (16%), TREK-1-like 112-pS (12%), and TRAAK-like 73-pS (3%) channels. In DRG neurons, mRNAs of all four K(2P) channels, as well as those of TASK-1 and TASK-3, were expressed, as judged by RT-PCR analysis. Our results show that TREKs and TRESK together contribute >95% of the background K(+) conductance of DRG neurons at 37 degrees C. As TREKs and TRESK are targets of modulation by receptor agonists, they are likely to play an active role in the regulation of excitability in DRG neurons.