Gap junction redistribution and reduced expression, a phenomenon termed gap junction remodeling (GJR), is often seen in diseased hearts and may predispose towards arrhythmias. We have recently shown that short-term pacing in the mouse is associated with changes in connexin43 (Cx43) expression and localization, but not with increased inducibility into sustained arrhythmias. We hypothesized that short-term pacing, if imposed on murine hearts with decreased Cx43 abundance, could serve as a model for evaluating the electrophysiologic effects of GJR. We paced wildtype (normal Cx43 abundance) and heterozygous Cx43 knockout mice (Cx43^+/-, 66% mean reduction in Cx43) for six hours at 10-15% above their average sinus rate. We investigated the electrophysiologic effects of pacing on the whole animal using programmed electrical stimulation, and in isolated ventricular myocytes with patch clamp studies. Cx43^+/- myocytes had significantly shorter action potential durations (APD) and increased steady state and inward rectifier potassium currents (Iss and IK1, respectively) compared to wildtype littermate cells. In Cx43^+/- hearts, pacing resulted in significant prolongation of ventricular effective refractory period and action potential duration, and significant diminution of Iss compared to unpaced Cx43^+/- hearts. However, these changes were not seen in paced wildtype mice. These data suggest that Cx43 abundance plays a critical role in regulating currents involved in myocardial repolarization and their response to pacing. Our study may aid in understanding how dyssynchronous activation of diseased, Cx43-deficient myocardial tissue can lead to electrophysiologic changes which may contribute to the worsened prognosis often associated with pacing in the failing heart.