Plasma and urinary levels of thiobarbituric acid reactive substances (TBAR) were determined in 24 hyperthyroid patients, 19 hypothyroid subjects, 35 controls, and 17 hyperthyroid patients before and after propylthiouracil (PTU) treatment (400 mg/day for 2-3 months), as indexes of lipid peroxidation. These measurements were carried out together with t-butyl hydroperoxide (t-BHP)-induced oxygen uptake and visible chemiluminescence in erythrocytes as functional tests related to the antioxigenic capacity of cells. Hyperthyroid patients exhibited increased levels of plasma and urinary TBAR compared to controls. Erythrocyte suspensions from hyperthyroid patients showed, compared to controls, higher rates of oxygen consumption with shorter induction periods upon addition of t-BHP, together with 142% and 75% increases in basal and t-BHP-induced chemiluminescence, respectively. Levels of TBAR in untreated hyperthyroid patients in plasma (16.2 +/- 1.3 pmol/mg of protein) and urine (15.9 +/- 1.5 nmol/mg of creatinine) were decreased after PTU treatment (Plasma, 9.5 +/- 0.7, p less than 10(-4); urine, 7.8 +/- 0.9, P less than 10(-5) to values not significantly different from those of the control group (plasma, 10.3 +/- 0.6; urine, 7.9 +/- 0.7). Compared to control, elevated rates of oxygen uptake induced by t-BHP, basal and t-BHP-induced chemiluminescence in erythrocyte suspensions from untreated hyperthyroid patients were reverted to normal by PTU, while decreased induction period (T0) values were enhanced. Determination of these lipid peroxidative parameters in hypothyroid patients revealed no significant changes over control values, excepted t-BHP-induced chemiluminescence in erythrocytes that was diminished. These data indicate that hyperthyroidism is associated with a pro-oxidant condition characterized by an enhancement in circulating and urinary lipid peroxidative indexes, which is suppressed by PTU treatment. It is suggested that this condition might reflect an oxidative stress at cellular level in tissues which are target for thyroid hormone action with a calorigenic response.