Ionic currents activated by extracellular adenosine 5′-triphosphate (ATP) were studied in voltage-clamped dorsal root ganglion neurons from rats and bullfrogs. Under quasiphysiological ionic conditions, ATP- activated current reversed near 0 mV and showed strong inward rectification. Strong inward rectification was maintained even in symmetric solutions of divalent-free Cs glutamate. Examined with a resolution of 10s of microseconds, the rectification was instantaneous. Inward current was greatly reduced when N-methyl-D-glucamine was substituted for external Na. ATP-activated inward currents could be recorded with Ca as the only external cation; estimated from reversal potentials, the ratio of Ca to Na permeability is about 0.3. Unitary channel activity could be recorded when ATP was applied to outside-out patches. When activated, a single channel flickered rapidly, with a mean current of about 0.5 pA at -100 mV. Large concentrations of ATP put the channel in the activated, flickery condition virtually all the time, while at lower concentrations, periods of flickering were interspersed with closures. Analysis of whole-cell current fluctuations showed precisely the characteristics expected if such channels underlie the macroscopic currents.