Background: The Long-QT Syndrome Type 3 (LQT3) is characterized by action potential duration (APD) prolongation due to cardiac voltage-gated sodium channel (Nav1.5) gain of function. Arrhythmia risk increases with age in LQT3 patients, but it is unknown whether this is due to progressive APD prolongation. Recent studies have shown that the LQT3 phenotype can be exacerbated by widening the perinexus, an intercalated disc nanodomain adjacent to gap junctions. However, it remains unclear if the perinexus mediates the LQT3 phenotype during aging. Objective: To investigate the role of the perinexus during aging in LQT3. Methods: Ventricular epicardial APD was obtained by optically mapping Langendorff-perfused adult (3-month) and aged (15-month) guinea pig hearts. LQT3 phenotype was induced with anemone toxin (ATXII), and the perinexus was widened with an intercalated disc adhesion inhibitor (βadp1). Perinexal width (Wp) was quantified by transmission electron microscopy. Susceptibility to arrhythmia was assessed with burst pacing in the absence and presence of βadp1 and evaluated from electrocardiograms (ECG). Results: APD was not different between adult and aged hearts at baseline, with ATXII, or ATXII+βadp1. Wp was significantly wider in adult LQT3 hearts either without or with βadp1 relative to aged hearts (without βadp1: 29.6±2.5 vs 22.9±3.8 nm; with βadp1: 38.3±1.7 vs 29.9±1.2 nm). These data suggest the perinexus narrows with aging and attenuates previously reported cellular APD prolongation such that there is no difference in whole-heart, epicardial APD between adult and aged LQT3 hearts. However, ECG Tpeak-Tend interval was not prolonged in adult, but significantly prolonged in aged hearts after treatment with ATXII+βadp1 (18.7±5.6 vs 35.5±11.5 ms), suggesting that βadp1 increases transmural dispersion of repolarization during aging in LQT3. Furthermore, ATXII+βadp1 did not induce arrhythmias in adult (0/6) but was arrhythmogenic in aged hearts (3/5). Conclusions: Perinexal narrowing associated with aging attenuates APD prolongation and reduces arrhythmic risk in aged LQT3 hearts. Loss of perinexal adhesion in aged LQT3 hearts results in significant and heterogeneous APD prolongation and increased risk of arrhythmia.