Background Adult females are more prone to Torsade de Pointes (TdP) than males in long QT type 2 (LQT2) elicited by inhibiting the K + current, I Kr. Sex-differences in the propensity to early afterdepolarizations (EADs) and TdP are reversed in adolescence and correlate with higher levels of L-type Ca 2+ channels and current, I Ca,L at the base of the epicardium in adolescent male rabbit hearts, even though female hearts have longer action potential (AP) durations. Higher Ca 2+-influx via I Ca,L should be balanced by higher Ca 2+-efflux. Objectives Sex-differences in Na +-Ca 2+ exchanger and its current, I NCX are investigated in adolescent rabbit hearts to determine if they parallel sex-differences in I Ca,L. Methods and Results Optical APs and intracellular Ca 2+ (Ca i) were mapped in pre-pubertal Langendorff rabbit hearts. E4031 (0.5μM) blocked I Kr, elicited EADs and TdP in male (n=10) but not female (n=18) hearts. KB-R4379 (I NCX inhibitor, 1μM) reversed (n=5/5) or protected from EADs if perfused before E4031 (n=6/6). I NCX density was 20-30% greater in base compared to apex myocytes of pre-pubertal male (p<0.02) or to apex and base myocytes of female hearts (p<0.02). In Western blots, NCX was 22% higher at the base of male versus female hearts and 25% higher than the apex (n=4, each gender). Immuno-histochemistry of ventricular sections revealed higher NCX levels in male versus female hearts. Conclusions Sex and regional differences in NCX correlate with L-type Ca 2+ channels and NCX inhibition suppresses EADs and TdP in LQT2. These data provide compelling evidence that sex-differences in Ca 2+ handling determine the arrhythmia phenotype.