1. When a steady dose of acetylcholine (ACh) is applied to an end-plate, the resulting depolarization is accompanied by a significant increase in voltage noise.2. The characteristic properties of this ACh noise (amplitude and time course) are examined under various experimental conditions. The voltage noise is analysed on the assumption that it arises from statistical fluctuations in reaction rate, and in the frequency of the elementary current pulses (;shot effects') produced by the action of ACh molecules.3. The elementary ACh current pulse (amplitude approximately 10(-11) A), arises from a conductance change of the order of 10(-10) Omega(-1) which lasts for approximately 1 ms (at 20 degrees C), and produces a minute depolarization, of the order of 0.3 muV. It is associated with a net charge transfer of nearly 10(-14) C, equivalent to approximately 5 x 10(4) univalent ions.4. At low temperature, and during chronic denervation, the duration of the elementary current pulse increases, and the elementary voltage change becomes correspondingly larger.5. Curare has little or no effect on the characteristics of the elementary event.6. A comparative study of ACh and carbachol actions shows that carbachol produces considerably briefer, and therefore less effective, current pulses than ACh.