OBJECTIVE: To investigate the dynamic changes of transforming growth factor-β_1 (TGF-β_1) mRNA and protein expression and their effects on lung development in premature rats with chronic lung disease (CLD). METHODS: CLD was induced by hyperoxia exposure (FiO_2=0.90) in neonatal premature rats. The dynamic changes of lung coefficient and radical alveolar counts (RAC) were observed; and the expressions of TGF-β_1 mRNA and protein were assayed with reverse transcription polymerase chain reaction (RT-PCR), immunohistochemical and image processing technique on days 1, 3, 7, 14 and 21 after birth. For comparison analysis, 30 premature rats exposed to air were used as the Control group. RESULTS: There were no differences in the lung coefficient and the RAC between the CLD and the Control groups within 3 days after birth. However on the 7th and 14th days, the lung coefficient and the RAC of the CLD group were significantly lower than those of the Control group. The RAC decreased to the nadir on the 21st day, but the lung coefficient was not different from the Control group. In the CLD group, the expression level of TGF-β_1 mRNA increased on the 3rd day, peaked on the 14th day and remained higher on the 21st day; the higher expression of TGF-β_1 protein was detected on the 7th day and peaked on the 21st day; and the TGF-β_1 protein expression was negatively correlated with RAC(P =0.0027). CONCLUSIONS: The over-expression of TGF-β_1 in lung tissues is associated with the lung development disorder. The TGF-β_1 expression level may be an index for the assessment of lung development.