Single channels activated by externally applied ATP were investigated in cultured sensory neurons from nodosal and spinal ganglia of rat using patch clamp and concentration clamp methods. Mean conductance of single ATP-activated channels was 17 pS when measured at a holding potential of -75 mV in saline containing 3 mM Ca2+ and 1 mM Mg2+. Sublevels of conductance were detected in some cases. The current-voltage relationship for a single channel is highly non-linear and demonstrates inwardly directed rectification. The I-V curve obtained for single channels was identical to that for macroscopic current. ATP activated the channels in the absence of divalent cations (in ethylenediaminetetra-acetate-containing medium) as well as in their presence. This indicates that ATP as a free anion can activate the receptor. Ca2+ ions decreased both macro- and microscopic ATP-activated currents. The concentration dependence of this Ca2+ effect does not fit a single site binding isotherm. The single channel current demonstrated prominent fluctuations. When measured in the 0-4 kHz frequency band the amplitude of fluctuations evaluated as a double r.m.s. was about 30% of the mean amplitude of current. The autocorrelation function for the current fluctuations in an open channel could be approximated by a single exponential with the time constant of 0.4 ms. These fluctuations did not depend on the presence of divalent cations in the external medium. The open time distribution for the investigated channels could be described by a sum of two exponentials. Presumably this reflects the existence of two subtypes of ATP-activated channels.