In the present study, we found that histologically andbiochemically maximum effect of radiation pneumonitis occurredafter 2 days of exposure to irradiation, while damage started in ratswhich were sacrified immediately after irradiation but still less thanthat occurs after 2 days, compared to rats which were sacrified 7days after irradiation where pneumonitis persisted but at the sametime repair startedOur study proved the effect of dexamethasone to decrease lipidperioxidation through decreasing the levels of MDA and LDH.Group of rats which took dexamethasone before irradiation andcontinued it after gave better results both histologically andbiochemically even abolish the pneumonitis induced by radiation.