Abstract: Recent studies have shown that long-term cocaine use induces diastolic impairment and a myocardial oxidative stress. Recently, we have reported that cocaine-induced cardiac dysfunction may be due to a mitochondrial reactive oxygen species (ROS) overproduction, which occurs at the same time as xanthine oxidase (XO) activation. In this work, we hypothesized that XO activation contributes to mitochondrial ROS overproduction, which in turn contributes to diastolic dysfunction. To test this, we used a well-established in vivo model of cocaine-induced diastolic dysfunction. In this experimental model treated with or without allopurinol, an inhibitor of XO, we measured mitochondrial ROS production and function. Mitochondrial alterations were characterized by an increase in oxygen consumption through complexes I and III, a reduction in ATP production, and an increased ROS production specifically in isolated interfibrillar mitochondria. Allopurinol treatment prevented the rise in mitochondrial ROS levels and the decrease in ATP production. In the same way, allopurinol treatment improved ventricular relaxation with a decrease in Tau, an index of left ventricle relaxation and of end-diastolic pressure volume relation. These results confirmed the critical role of XO in the sequence of events leading to cocaine-induced cardiac dysfunction.