In the first section of this short review the change of the cardiac action potential (APD) with the rate of stimulation under physiological conditions is described and mechanistically analyzed. A fast phase of adaptation is mainly caused by changes in gating characteristics of ionic currents, and rapid modulation of the Na+/Ca2+ exchanger. The slower phase is largely conditioned by incomplete recovery from inactivation of the late Na+ current (late INa) and changes in ion concentrations of [K+]e, [Na+]i, and [Ca2+]i, which cause secondary changes in the permeation and the gating of ion channels and flux through transporters. In a second section, an analysis is presented of the rate dependence of APD in pathological conditions and its importance in the genesis of arrhythmias in hypertrophy, heart failure, congenital, and acquired LQT syndromes is summarized. The role of the late INa, Na+, and Ca2+ overload is emphasized. Special attention is given to the paradoxical transient lengthening of APD in LQT3 syndrome for the sudden increase in rate in this setting. The third section consists of a short commentary on Na+ and Ca2+ overload and drugs which block the late INa.