Background: Chronic atrial fibrillation (cAF) is associated with abnormal atrial Ca 2+ signalling, causing remodeling, contractile dysfunction and triggered activity. The poorly understood molecular basis of impaired atrial Ca 2+ signaling in cAF was the object of this study. Methods: Membrane currents (whole-cell voltage clamp) and [Ca 2+ ] i (Fluo-3) were measured in right atrial myocytes from sinus rhythm (Ctl) and cAF patients. Protein expression was quantified by immunoblotting. Results: In cAF L-type Ca 2+ current density and [Ca 2+ ] i transient amplitude were 43% and 46% lower than in Ctl, whereas diastolic [Ca 2+ ] i was unchanged (Figure). Despite reduced protein expression of Serca2a, sarcoplasmic reticulum (SR) Ca 2+ content calculated by integrating Na + -Ca 2+ exchange current (I NCX ) during caffeine-induced Ca 2+ release was preserved in cAF, likely because of attenuated Serca2a inhibition by hyperphoshorylated phospholamban. The phosphorylation of ryanodine receptor channels at Ser2809 was 140% higher in cAF compared to Ctl, without protein changes in the major SR Ca 2+ buffer calsequestrin. The decay of the caffeine-evoked Ca 2+ transient is attributable to Ca 2+ transport by NCX and was 100% faster in cAF, consistent with the greater I NCX amplitude in cAF. In addition, a given Ca 2+ release produced larger INCX current, possibly as a result of NCX protein upregulation. Conclusions: We conclude that reduced SR Ca 2+ release and enhanced NCX activity both contribute to atrial hypocontractility in cAF. The generation of a larger depolarising I NCX for a given SR Ca 2+ release may cause delayed afterdepolarisations and triggered activity, contributing to AF maintenance.