High resolution measurements of the current through individual ion channels activated by acetylcholine (AChR- channels) in frog muscle have shown that these currents are discrete pulse-like events with durations of a few milliseconds. Fluctuation and relaxation measurements of end-plate currents have led to the conclusion that the rate of channel opening increases with agonist concentrations, and that the channel, once open, closes spontaneously. Katz and Thesleff have shown, however, that in the continued presence of ACh, the initial end-plate current declines to an equilibrium value with a time constant of several seconds. This reversible phenomenon is referred to as receptor desensitization. We report here that in the presence of ACh concentrations sufficient to cause desensitization, single channel current pulses appear in groups. From the temporal sequence of the pulses, we have derived estimates of the rates of activation and desensitization of the AChR-channel.