Physiol. 25 (2): 191-196, 1968.-The cardiac action-potential duration(A) is influenced by the preceding diastolic interval (Do) and inversely affects the next diastolic interval(D]). Action potentials were recorded through microelectrodes impaled into frog ventricular muscle strips driven electrically to produce alternation. The graph of A= f (Do) during the steady state was plotted and the relationship D1= f (A) was drawn. The point of intersection of these two curves describes the action-potential duration and diastolic interval in the steady state. When stimulus frequency is altered, the plots of action potentials immediately following the rate change deviated from the steady-state curve. The action-potential behavior in this nonsteady state was explicable by a functional curve, the slope of which determined the occurrence of alternans. Transient alternation appeared and lasted longer as this slope increased and at a driving rate where the slope was+ 1, persistent akernans occurred. The existence of more than one amplitude of alternation at a given rate was also deduced from the graphs and demonstrated experimentally. rate effect on intracellular cardiac action potentials; negative feedback; cardiac electrical alternans; arrhythmias; homeostasis; nonsteady-state behavior; diastolic interval