Potassium bromate (KBrO3) is a prooxidant and carcinogen. Melatonin is a highly effective antioxidant. Indole-3-propionic acid (IPA; indole substance) and propylothiouracil (PTU; antithyroid drug) reveal some antioxidative effects. The aim of the study was to evaluate KBrO3-induced lipid peroxidation (LPO) in vitro in tissues collected from control or melatonin-treated rats, and to compare potential preventive effects of melatonin, IPA and PTU. Kidney, liver and lung homogenates from either control or melatonin-pretreated rats (0.0645 mmol/kg b.w., i.p., twice daily, 10 days) were incubated in the presence of KBrO3 (0.1, 0.5, 1.0, 2.5, 5.0, 10.0 mM). Then, control lung homogenates were incubated with KBrO3 (10.0 mM) together with melatonin (0.01, 0.1, 0.5, 1.0, 5.0, 7.5 mM), or with IPA or with PTU. LPO products (malondialdehyde+4-hydroxyalkenals) were measured spectrophotometrically. Melatonin injections prevented KBrO3-induced LPO in lung homogenates. Melatonin, IPA and PTU, used in vitro, reduced KBrO3-induced LPO in control lungs. Unexpectedly, KBrO3 caused a concentration-dependent decrease in LPO in liver and kidney homogenates from control but not from melatonin-treated rats. Potassium bromate-induced LPO in the rat lung homogenates suggests that the lung may be the target for this carcinogen. An exposure of organisms to melatonin decreases tissue sensitivity to KBrO3-induced damage, possibly by restoring the oxidative balance.