Introduction. Pulmonary edema, reduced pulmonary gas exchange and systemic hypoxia can be induced or exacerbated by hyperoxia (high FiO2). Systemic hypoxia or prolonged hyperoxia have both been associated with bradycardia. Hence, hyperoxia‐induced bradycardia, pulmonary edema and systemic hypoxia may be related. The purpose of this study was to assess the contribution of hypoxia to known hyperoxia‐induced bradycardia.Methods. Male C57BL/6J mice (8‐12 wks; 20‐30g; n = 8‐10 per group) were exposed to hyperoxia (100% O2), hypoxia (10% O2) or room air for 55 hours while ECG was recorded. Heart rate (HR), QT‐interval (QTcB) and PR‐interval (PR‐I) responses were calculated from the ECG waveforms.Results. 55hrs hyperoxia induced a substantial reduction in HR and QTcB, compared to baseline (~365bpm HR; ~98ms QTcB; P<0.05), air controls and hypoxia‐exposed mice. Further, an increase in PR‐I, compared to baseline (~19ms; P<0.05), was found in the hyperoxia exposed mice only, to which increases in P‐wave duration contributed the most.Conclusion. This study suggests that if prolonged hyperoxia induces systemic hypoxia, the hypoxia does not contribute to the established hyperoxia‐induced bradycardia.Grant Funding Source: Supported by UNC Charlotte