Administration of daily doses of 0.1 mg of 3,5,3'-triiodothyronine (T3)/kg body weight for 3 consecutive days to fed rats elicited a calorigenic response in the animals, in concomitance with a 36% increase in the rate of 0(2) consumption by the liver. In these conditions, liver submitochondrial particles (SMP) from T3-treated rats exhibited marked increases in the rate of superoxide radical generation, both in the presence of NADH (142%) or succinate (152%). Furthermore, liver SMP from hyperthyroid animals released hydrogen peroxide at higher rates than those of euthyroid rats, either under basal conditions or in the succinate-supported process, both in the absence and presence of antimycin-A. It is concluded that the hyperthyroid state in the rat leads to a drastic enhancement in the capacity of liver mitochondria to produce active oxygen species, which correlates with the elevated respiratory rate observed in the intact organ.