In the present study, we investigated the radioprotective effect of cyclo(L-phenylalanyl-L-prolyl) on irradiated rat lungs to determine its potential as a radioprotective agent. We found that early lung damage induced by irradiation was reduced by treatment with 40mg/kg of cyclo(L-phenylalanyl-L-prolyl) in the latent and early pneumonitis phases. Expression of $TNF-{alpha};and;TGF-{eta}1$ at 2 and TGF-${eta}1$ at 8 weeks post-irradiation was decreased in animals that received both radiation and cyclo(L-phenylalanyl-L-prolyl) compared with animals that received radiation alone. Evidence indicated that the proinflammatory cytokine TNF-${alpha}$ and the fibrogenic cytokine TGF-${eta}1$ likely play a role in the radioprotective effect of cyclo(L-phenylalanyl-L-prolyl). However, besides TNF-${alpha}$ and TGF-${eta}1$ expressions, the precise mechanism by which cyclo(L-phenylalanyl-L-prolyl) ameliorates the induced radiation damage is not clear.