Mitochondrion is the main production site for reactive oxygen species (ROS). In endotherms, the existence of a positive relationship between ROS production and metabolic rate is acknowledged. But, little is known about ectotherms, especially fish, with a metabolic rate dependent on the environmental temperature. The maximal oxygen consumption and the production of highly reactive hydroxyl radicals by permeabilized red muscles of yellow and silver eels and trouts were measured concomitantly and compared to those of rats chosen for their comparable body mass, but different metabolic rate. The positive correlation found in fish between the metabolic rate and the ROS production showed a shift with respect to mammals.