--- - |2- Hyperoxia is a common intervention in ICU. It is known that sex differences do exist in cardiovascular physiology, and this manuscript describes the sex differences in hyperoxia induced cardiac pathophysiology. Hyperoxia (>90% oxygen) is commonly implemented in mechanically ventilated patients. Reports suggest that hyperoxia is directly associated with in‐hospital mortality in ventilated patients. Certain studies also show that mortality in women undergoing mechanical ventilation is significantly higher than that in men. Additionally, females are predisposed to certain cardiac electrophysiological risks, including QTc prolongation. In this study, we assessed the impact of hyperoxia in male and female mice (C57BL/6J) at age 8–10 weeks. On completion of either hyperoxia or normoxia exposures, physical, hemodynamic, biochemical, functional, electrophysiological, and molecular assessments were conducted. Hyperoxia‐exposed mice lost a significant amount of body mass, compared with normoxia controls, in both sexes. However, while both genders developed brady‐arrhythmia after hyperoxia exposure, female mice exhibited significantly reduced heart rates compared with males, with significantly elevated RR intervals. Additionally, 50% mortality was observed in females, whereas no mortality was reported in males. Furthermore, unlike in male mice, we observed no hypertrophy upon hyperoxia exposure in female mice. We reported that both hyperoxia‐treated male and female mice exhibit significant hyperdynamic left ventricular ejection fraction, which is marked by % ejection fraction > 70 compared with the normoxia controls. We also noted significant reductions in stroke volume and cardiac output in both mice with hyperoxia. Surface ECG also demonstrated that hyperoxia exposure significantly augments RR, PR, QRS, QTc, and JT intervals in both sexes. Molecular analysis of left ventricular tissue demonstrated dysregulation of potassium ion channels in hyperoxia‐treated males and females. In summary, we determined that sex differences are present with 72 hr hyperoxia exposure. - 'Journal of Cellular Physiology, Volume 234, Issue 2, Page 1491-1501, February 2019. '