The electrophysiological mechanisms by which changes in [K+]o suppress and initiate pacemaker activity were studied in guinea-pig isolated sino-atrial node (SAN) superfused in vitro. High [K+]o (10 mM or higher) gradually decreases maximum diastolic potential and action potential amplitude, until only subthreshold responses and eventually quiescence follow. When the threshold potential is missed, an oscillatory afterpotential (Vos) is often superimposed on early diastolic depolarization (DD). During the subsequent late DD, gradually increasing oscillatory prepotentials (ThVos) appear, whose depolarizing phase may initiate an action potential. If ThVos miss the threshold, they gradually decrease in size. In quiescent SAN, on decreasing high [K+]o, the resumption of spontaneous activity is caused by ThVos. In high [K+]o, Cs+ and Ba2+ may induce spontaneous activity in quiescent SAN and accelerate spontaneously active SAN. A low [Ni2+]o does not suppress SAN, whereas nifedipine blocks excitation (but not DD); and high [Ca2+]o induces spontaneous discharge in quiescent SAN. Tetrodotoxin and low [Na+]o often cause block of conduction. In conclusion, high [K+]o suppresses SAN discharge not by abolishing DD, but by preventing the attainment of the threshold. A slower rhythm may be maintained by ThVos arising during the late DD. After arrest, resumption of activity is due to gradually increasing ThVos. The effects of current blockers suggest that in high [K+]o the mechanism underlying DD may involve IK, but not I(f) or ICa. Initiation of discharge by high [Ca2+]o and induction of quiescence by nifedipine suggest a role of Ca2+ in excitation (but not in DD). The effects of tetrodotoxin and low [Na+]o suggest a role of Na+ in conduction within SAN superfused in high [K+]o.