Activities of two antioxidant enzymes, superoxide dismutase (SOD) and glutathione peroxidase (GPx) were determined in plasma of rats exposed to tobacco smoke (500, 1000 and 1500 mg CO/m3 air in 5 days) and to 1000 mg CO/m3 in 1, 2 and 3 weeks. When compared to controls, the SOD activity was decreased in the exposed animals, the lowest smoke dose gave the most pronounced effect and the highest dose revealed the least diminution. GPx activity was statistically higher in the rats inhaled with the highest smoke dose. Time of exposure did not effect GPx but SOD raised in correlation to week number, although it did not reach control values over the experiment period. The observed alterations in plasma probably mirror the organ adaptation to oxidative stress in animals exposed to tobacco smoke.