by letting their membrane potential fall to the level where an action potential will start which subsequently activates surrounding cells to fire an action po­ tential. The first question which is tackled in this book is which processes underly this spontaneous diastolic depolarization. This is discussed in section I, concerning the fundamental properties of pacemaker cells with special refer­ ence to ionic membrane currents. Although views still quite differ about the exact nature of the membrane processes that cause the automatic pacemaker dis­ charge there is agreement that diastolic depolarization is brought about by the interaction of a number of ionic current systems, including both inward and out­ ward going currents.