The effect of thinner inhalation for five weeks on lipid peroxidation (TBA-RS, thiobarbituric acid Reducing Substances) levels in rat whole brains was studied. Also superoxide dismutase (SOD) activities and reduced glutathione (GSH) levels were examined. Significant increases according to the time of thinner inhalation were found in TBA-RS levels. While TBA-RS exceed their normal levels (1.95±0.05, 1.98±0.03, 2.08±0.05, 2.20±0.05, 2.48±0.05 and 3.08±0.06 nmol/mg protein, control, first, second, third, fourth and fifth weeks, respectively), SOD activities were found to be decreased from the second inhalation week (12.50±1.22, 11.75±1.37, 10.58±1.10, 9.36±0.80, 8.43±0.81 and 8.17±0.71 U/mg protein, control, first, second, third, fourth and fifth weeks respectively). Significant reductions in GSH levels, beginning from the third inhalation week were also found (1.60±0.05, 1.56±0.05, 1.49±0.05, 1.23±0.06, 0.91±0.03 and 0.85±0.04 (μmol/g wet weight, control, first, second, third, fourth and fifth weeks respectively).