The effects of sex and training on mitochondrial function and its sensitivity to in vitro reactive oxygen species (ROS) exposure were investigated in male and female silver European eels. The critical swimming speed (Ucrit) was used as an index of training intensity and of overall swimming performance. After four days of training (10 h at 70% and 14 h at 50% of their Ucrit before training), Ucrit increased significantly in trained eels for both sexes. In untrained eels, basal mitochondrial respiration (V0) measured in the absence ofADP in permeabilized cardiac fibreswas significantly higher and more resistant to in vitro ROS exposure in males than in females. No difference in ATP production (VATP) or hydroxyl radical release (V●OH ) was observed between male and female eels. In trained male eels, V●OH decreased in red muscle fibres concomitantly to a trend in an increased ATP production. In trained female eels, a different trend in ATP decrease was observed. Training significantly enhanced respiratory rate Vmax and resistance to in vitro ROS exposure in cardiac fibres of male silver eels but not females. In male silver eels, but not females, training could induce an improvement of energy efficiency in red muscle and a greater resistance of mitochondrial function to ROS exposure in cardiac fibres. These beneficial effects could contribute to the highly efficient swimming performance of the male eel, which could serve to compensate their smaller size, helping them to achieve their transoceanic migration and synchronized arrival with the females at their reproduction area.