Superoxide/hydrogen peroxide production by rat skeletal muscle mitochondria oxidising glycerol-3-phosphate alone (reverse electron transport (RET), site I_Qr) or in the presence of 50 µM glutamate plus malate (forward electron transport (FET), site I_Qf), measured in plate-reader assays in the presence of 2 µM atpenin A5 to prevent electron backflow into site II_F and 10 µM S3QEL3 to suppress site III_Qo. Grey bars show overall measured rates of hydrogen peroxide production; orange and blue bars stacked within the grey bars show the rates suppressed by the addition of 1 µM S1QEL2.1. Values are means ± SEM (n = 3 independent experiments each the mean of at least three technical replicates).