Arrhythmias following cardiac stress are a key predictor of death in healthy population. Carbon monoxide (CO) is a ubiquitous pollutant promoting oxidative stress and associated with hospitalization for cardiovascular disease and cardiac mortality. We investigated the effect of chronic CO exposure on the occurrence of arrhythmic events after a cardiac stress test and the possible involvement of related oxidative stress. Wistar rats exposed chronically (4 weeks) to sustained urban CO pollution presented more arrhythmic events than controls during recovery after cardiac challenge with isoprenaline in vivo. Sudden death occurred in 22% of CO-exposed rats versus 0% for controls. Malondialdehyde (MDA), an end-product of lipid peroxidation, was increased in left ventricular tissue of CO-exposed rats. Cardiomyocytes isolated from CO-exposed rats showed higher reactive oxygen species (ROS) production (measured with MitoSox Red dye), higher diastolic Ca(2+) resulting from SR calcium leak and an higher occurrence of irregular Ca(2+) transients (measured with Indo-1) in comparison to control cells after a high pacing sequence. Acute treatment with a ROS scavenger (N-acetylcysteine, 20 mmol/L, 1 h) prevented this sequence of alterations and decreased the number of arrhythmic cells following high pacing. Chronic CO exposure promotes oxidative stress that alters Ca(2+) homeostasis (through RYR2 and SERCA defects) and thereby mediates the triggering of ventricular arrhythmia after cardiac stress that can lead to sudden death.