Intracellular chloride activities have been measured in muscle fibres with chloride-sensitive electrodes. It was found that the depolarization induced by carbachol produces an influx of chloride ions, the accumulation of which leads to further sustained depolarization. The present study thus confirms the evidence put forward by Jenkinson & Terrar (1973) of an influence of chloride ions on the membrane potential after a prolonged application of carbachol.