OBJECTIVE To investigate the effect of N-acetylcysteine (NAC) on apoptosis of pneumocytes and expression of caspase-3 during lung ischemia/reperfusion injury (LIRI) in rats, and to explore the possible role of NAC in pneumocyte apoptosis. METHODS Forty-two male Sprague-Dawley rats were randomly divided into three groups: sham operation group, LIRI group (LIRI was produced by 45 minutes of clamping of the pulmonary hilum followed by 3 hours or 6 hours of reperfusion), and NAC group (NAC 150 mg/kg was injected intraperitoneally before LIRI). Lung specimens were harvested 3 hours or 6 hours after LIRI. Apoptosis rate in lung tissue was determined with flow cytometer after Annexin V-fluorescein isothiocyanate (FITC) and propidium iodide (PI) staining. Malondialdehyde (MDA, thiobarbituric acid) and superoxide dismutase (SOD, xanthine oxidase) of lung tissue were measured. Expression of caspase-3 in lung was determined by reverse transcription-polymerase chain reaction (RT-PCR), and the changes in ultrastructure of lung tissue were observed by electron microscope. RESULTS Compared with that of the sham operation group, apoptosis rate of pulmonary cells was significantly increased at 3 hours and 6 hours in LIRI group [(25.60 ± 3.22)% vs. (2.19 ± 0.48)% , (26.01 ± 4.50)% vs. (2.55 ± 0.36)%], the content of MDA (nmol/mg) was significantly increased (3.26 ± 0.32 vs. 0.73 ± 0.23, 3.53 ± 0.46 vs. 1.08 ± 0.42), and the activity of SOD (U/mg) was significantly lowered (32.80 ± 3.82 vs. 60.51 ± 6.81, 33.44 ± 3.24 vs. 64.19 ± 6.60), and the expression of caspase-3 mRNA in lung tissue was significantly up-regulated (0.717 ± 0.037 vs. 0.216 ± 0.046, 0.744 ± 0.046 vs. 0.227 ± 0.037, all P < 0.01). Compared with that of the LIRI group, apoptosis rate of pulmonary cell was significantly decreased [(14.42 ± 1.61)% vs. (25.60 ± 3.22)%, (10.02 ± 1.64)% vs. (26.01 ± 4.50)%], content of MDA (nmol/mg) was lowered significantly (1.75 ± 0.33 vs. 3.26 ± 0.32, 2.15 ± 0.25 vs. 3.53 ± 0.46), and activity of SOD (U/mg) was significantly elevated (42.76 ± 2.06 vs. 32.80 ± 3.82, 44.94 ± 3.11 vs. 33.44 ± 3.24, all P < 0.01) in NAC group. The expression of caspase-3 in lung tissue was remarkably down-regulated compared with that of LIRI group (0.441 ± 0.038 vs. 0.717 ± 0.037, 0.410 ± 0.037 vs. 0.744 ± 0.046, both P < 0.01). The ultrastructure changes in lung tissue were milder in NAC group than in LIRI group. Positive correlation was found between the expression of caspase-3 and apoptosis rate and the content of MDA (3 hours: r = 0.9036, 0.9216; 6 hours: r = 0.9655, 0.9650, all P < 0.01), but negative correlation was found between apoptosis rate and activity of SOD (3 hours: r = -0.9511, 6 hours: r = - 0.9574, both P < 0.01) after LIRI 3 hours and 6 hours. CONCLUSIONS During early period of LIRI, caspase-3 was significantly deregulated by NAC, therefore the cellular apoptosis was inhibited, thus protecting lung tissue from LIRI.