1. Here we review evidence obtained recently by us indicating that the poor longevity of isolated mammalian skeletal muscle preparations at temperatures in the normal physiological range is related to the increased production of reactive oxygen species (ROS) in the resting muscle. 2. Temperature-induced ROS production increases markedly above 32 degrees C in isolated, resting skeletal muscle and is associated with the gradual and irreversible functional deterioration of the muscle. 3. The majority of the temperature-induced muscle ROS originates in the mitochondria and acts on various sites involved in excitation-contraction coupling.