Airway ciliated cells express an ATP-gated P2X receptor channel of unknown subunit composition (P2Xcilia) which is modulated by Na+ and by long exposures to ATP. P2Xcilia was investigated by recording currents from freshly dissociated rabbit airway ciliated cells with the patch-clamp technique in the whole-cell configuration. During the initial continuous exposure to extracellular ATP, P2Xcilia currents gradually increase in magnitude (priming), yet the permeability to N-methyl-D-glucamine (NMDG) does not change, indicating that priming does not arise from a progressive change in pore diameter. Na+, which readily permeates P2Xcilia receptor channels, was found to inhibit the channel extracellular to the electric field. The rank order of permeability to various monovalent cations is: Li+, Na+, K+, Rb+, Cs+, NMDG+ and TEA+, with a relative permeability of 1.35, 1.0, 0.99, 0.91, 0.79, 0.19 and 0.10, respectively. The rank order for the alkali cations follows an Eisenman series XI for a high-strength field site. Ca2+has been estimated to be 7-fold more permeant than Na+. The rise in [Ca2+]i in ciliated cells, induced by the activation of P2Xcilia, is largely inhibited by either Brilliant Blue G or KN-62, indicating that P2X7 may be a part of P2Xcilia. P2Xcilia is augmented by Zn2+ and by ivermectin, and P2X4 receptor protein is detected by immunolabelling at the basal half of the cilia, strongly suggesting that P2X4 is a component of P2Xcilia receptor channels. Taken together, these results suggest that P2Xcilia is either assembled from P2X4 and P2X7 subunits, or formed from modified P2X4 subunits.