1. The clusters composed of 10-20 cells of the rabbit sinoatrial node were prepared by coronary perfusion of collagenase and their response to ionophoretic application of ACh was recorded. The hyperpolarizing response showed a sigmoidal onset without any clear latency when the ACh pipette was positioned close to the cell surface. 2. Equations describing the kinetics and amplitude of the ACh-induced K current were determined based on the reported experimental results obtained by relaxation and noise analysis. 3. The kinetics of the ACh-induced K current simulated well the time course of the ACh response and also the effect on the spontaneous action potential when data on the kinetics was incorporated into the S-A node pacemaker mathematical model.