By analysis of whole cell membrane currents in Na⁺ absorbing H441 human airway epithelial cells we have identified a K⁺ conductance (G_K) resistant to Ba²⁺ but sensitive to bupivacaine or extracellular acidification. In polarised H441 monolayers we have demonstrated that bupivacaine, lidocaine and quinidine inhibit basolateral membrane K⁺ current (I_Bl) whilst Ba²⁺ has only a weak inhibitory effect. I_Bl was also inhibited by basolateral acidification and, although subsequent addition of bupivacaine caused a further fall in I_Bl, acidification had no effect after bupivacaine demonstrating that cells grown under these conditions express at least two different bupivacaine-sensitive K⁺ channels, only one of which is acid sensitive. Basolateral acidification also inhibited short circuit current (I_SC) and basolateral bupivacaine, lidocaine, quinidine and Ba²⁺ inhibited I_SC at concentrations similar to those needed to inhibit I_Bl, suggesting that the K⁺ channels underlying I_Bl are part of the absorptive mechanism. Analyses using the reverse transcriptase / polymerase chain reaction showed that mRNA encoding several two pore domain K⁺ channels (K2P) was detected in cells grown under standard conditions (TWIK-1, TREK-1, TASK-2, TWIK-2, KCNK7, TASK-3, TREK-2, THIK-1 and TALK-2). We therefore suggest that K2P channels underlie G_K in unstimulated cells and so maintain the driving force for Na⁺ absorption. Since this ion transport process is vital to lung function, K2P channels thus play an important but previously undocumented role in pulmonary physiology.