Background: This experimental study investigated the protective effects of N-acetylcysteine (NAC) on peroxidative changes in fetal lungs in the offspring of rats exposed to cigarette smoke. Methods: Thirty fetal rats used for analysis, were divided into three groups as follows: control group (n = 10), whose mothers were exposed to fresh air; group I (n =10), whose mothers were exposed to cigarette smoke; and group II (n =10), whose mothers were exposed to cigarette smoke and given 10 mg/kg per day NAC. In groups I and II, smoke exposure was started 4 weeks before the pregnancy, and continued to the 14th day of pregnancy, and in Group II, NAG was administered intraperitoneally for 14 days. The mothers and their fetuses were decapitated on the 14th day of pregnancy. Malondialdehyde (MDA) and glutathione (GSH) levels were determined in the lung tissues of fetuses to determine the oxidantantioxidant balance. Results: While tissue MDA levels in Group I were found significantly higher than the control group (129.7 +/- 65.4 versus 63.4 +/- 15.4nmol/100mg protein, P < 0.05), GSH levels were significantly lower (17.1 +/- 7.3 versus 45.4 +/- 8.1 nmol/mg protein, P < 0.01). Furthermore, in Group II, MDA levels were significantly lower (56.9 +/- 20.6 versus 129.7 +/- 65.4 nmol/100 mg protein, P < 0.05), and GSH levels were significantly higher (34.57 +/- 10.7 versus 17.1 +/- 7.3 nmol/mg protein, P < 0.0001) when compared with Group L No statistically significant difference was found in tissue MDA and GSH levels between Group II and the control group (P > 0.05). Conclusions: These results suggest that smoke exposure during pregnancy causes oxidative damage in fetal lungs. This smoke-induced damage might be prevented by NAC.