The kinetics of activation and deactivation of P2X7-dependent single channel currents were investigated by means of the patch clamp method in the outside-out configuration. A combination of the U-tube technique and a piezo-driven liquid filament switch technique enabled the application and withdrawal of different concentrations of the agonist, ATP^4-, in less then 1 ms. Three different types of kinetics were observed and approximated by single exponential functions. A low-noise current component activated and deactivated with time constants of about 300 ms. Its amplitude was dependent on the concentration of ATP^4-and on the seal resistance but was found to be independent of the expression of human P2X7 receptors. It was attributed to non-specific effects of ATP on the seal resistance. Single-channel currents with a conductance of about 10 pS activated with activation and deactivation time constants of about 15 ms. The channel activation was only little dependent on the agonist concentration. Activation and deactivation were independent of the membrane voltage. We conclude that binding of ATP^4-is not the only rate limiting step in the activation of P2X7 channels and that the ATP^4-binding site is not located in the electrical field of the membrane. The third, noisy and slowly activating component could be attributed to a high affinity activation site on the P2X7 receptor.