Voltage clamp experiments were carried out in order to study the mechanism of the ACh action in the rabbit S-A node cell. The following results were obtained: 1. The reversal potential of the ACh-induced current behaved like a potassium electrode, confirming that the ACh-operated channels pass potassium ions selectively. 2. On depolarizing voltage jumps the ACh-induced current showed an instantaneous peak from which the current decayed to a new steady level (relaxation). On hyperpolarizing voltage jumps the initial step change in current was followed by a gradual increase. 3. The time course of the current change on voltage jumps was well fitted by a single exponential and the time constant became longer as the membrane potential was increased. 4. The instantaneous I-V curve was linear while in the steady state the curve became flatter at low negative membrane potentials and steeper at high negative membrane potentials. The results suggest that ACh opens a specific potassium channel when the drug is bound to the muscarinic receptor. The opening and closing rate constants for this potassium channel depend on the membrane potential in such a way that on depolarizing voltage jumps the fraction of open channels gradually decreases and on hyperpolarization the fraction increases.