This study investigated the effects of hyperoxia on dynamic changes of thioredoxin-1 (Trx1) and thioredoxin reductase-1 (TrxR1) in alveolar type. epithelial cells (AEC II) of premature rats. Pregnant Sprague-Dawley rats were sacrificed on day 19 of gestation. AEC. were isolated and purified from the lungs of premature rats. When cultured to 80% confluence, in vitro cells were randomly divided into air group and hyperoxia group. Cells in the hyperoxia group were continuously exposed to 95% O-2/5% CO2 and those in the air group to 95% air/5% CO2. After 12, 24 and 48 h, cells in the two groups were harvested to detect their reactive oxygen species (ROS), apoptosis, TrxR1 activity and the expressions of Trx1 and TrxR1 by corresponding protocols, respectively. The results showed that AEC. exposed to hyperoxia generated excessive ROS and the apoptosis percentage in the hyperoxia group was increased significantly at each time points as compared with that in the air group (P<0.001). Moreover, TrxR1 activity was found to be markedly depressed in the hyperoxia group in comparison to that in the air group (P<0.001). RT-PCR showed the expressions of both Trx1 and TrxR1 mRNA were significantly increased in AEC. exposed to hyperoxia for 12 and 24 h (P<0.01), respectively. At 48 h, the level of Trx1 mRNA as well as that of TrxR1 mRNA in the hyperoxia group was reduced and showed no significant difference from that in the air group (P>0.05). Western blotting showed the changes of Trx1 protein expressions in the hyperoxia group paralleled those of Trx1 mRNA expressions revealed by RT-PCR. It was concluded that hyperoxia can up-regulate the protective Trx1/TrxR1 expressed by AEC. in a certain period, however, also cause dysfunction of the cytoplasmic thioredoxin system by decreasing TrxR1 activity, which may contribute to the progression of oxidative stress and cell apoptosis and finally result in lung injury.