Transjunctional voltage regulates cardiac gap junctional conductance, but the kinetics of inactivation were considered too slow to affect cardiac action potential propagation. Connexin43 (C x 43) is abundantly expressed in the atrial and ventricular myocardium and the rapid ventricular conduction tissues (ie, His-Purkinje system) of the mammalian heart and is important to conduction through these cardiac tissues. The kinetics of Cx43 voltage gating were examined at peak action potential voltages using simulated ventricular myocardial action potential waveforms or pulse protocols exceeding 100-mV transjunctional potentials. Junctional current responses approximate the action potential morphology but conductance calculations reveal a 50% to 60% decline from peak to near constant plateau values. Junctional conductance recovers during phase 3 repolarization and early diastole to initial values. The bases for these transient changes in junctional conductance are the rapid decay kinetics in tens of milliseconds at peak transjunctional voltages (V_j) of 130 mV and the gradual increase in junctional conductance as V_j returns toward 0 mV. The decay time constants change e-fold per 22.1 mV above the half-inactivation voltage for C x 43 gap junctions of ±58 mV. A realistic dynamic model for changes in junctional resistance between excitable and nonexcitable cells during cardiac action potential propagation was developed based on these findings. This dynamic model of cardiac gap junctions will further our understanding of the role gap junctions play in the genesis and propagation of cardiac arrhythmias. The full text of this article is available online at http://www.circresaha.org.