Cytokine release from irradiated cells has been postulated to start soon after irradiation preceding detectable clinical and pathological manifestation of lung injury. The expression of transforming growth factor beta (TGF beta), a fibrogenic and radiation-inducible cytokine, was studied from 1-16 weeks after the 15 and 30 Gray (Gy) of thoracic irradiation to rats. Thoracic irradiation caused an increase in TGF beta protein in bronchoalveolar lavage (BAL) fluid peaking at 3-6 weeks as compared to sham-irradiated control rats. Steady state TGF beta mRNA expression as shown by whole lung northern blot assay paralleled the TGF beta protein expression in BAL fluid. The peak of TGF beta protein increase in BAL fluid between 3 and 6 weeks coincided with the initial influx of inflammatory cells in BAL fluid, but preceded histologically discernable pulmonary fibrosis that was not apparent until 8-10 weeks after irradiation. In conclusion. TGF beta and mRNA and protein upregulation preceded the radiation-induced pulmonary fibrosis, suggesting a pathogenetic role in the development of radiation fibrosis.