The aim of this experimental study is to evaluate oxidant/antioxidant status and the end products of lipid peroxidation in the liver of rats exposed to subacut formaldehyde (FA) inhalation (4-weeks). Thirty male Wistar albino rats divided into 3 separate groups randomly (10 for each group). Rats were exposed to atmospheres containing 0, 10 and 20 ppm FA continuously (8 hours per day, 5 days per week). Reduced glutathione (GSH), malondialdehyde (MDA) and nitric oxide (NO) levels, as well as xanthine oxidase (XO), and myeloperoxidase (MPO) activities were measured in rat liver homegenates. The results showed a remarkable reduction of GSH content (10 and 20 ppm FA) and XO activity (20 ppm FA) in liver tissues from rats exposed to for 4- weeks (p<0.005, p<0.0001 and p<0.0001, respectively). MDA and NO levels did not change in both two groups (10 ppm and 20 ppm). These findings suggest that the antioxidant system of liver tissue is moderately impaired by excessive FA exposure. The GSH has an important influence especially on subacute FA liver toxicity. GSH-related cellular defensive mechanisms may be deppressed and susceptibility to oxidative damage may high in rats exposed to FA subacutely.