Simvastatin (20 mg/kg/day) significantly attenuates radiation-induced apoptosis in the thymi of C57BL/6 mice. Cell apoptosis in the thymus after irradiation was assessed by TUNEL assay with apoptotic nuclei stained brown and normal nuclei stained blue (A). Magnification, ×400. Apoptotic cells were significantly less in the radiation + simvastatin (RS) group compared to R group (A). Representative electron micrographs taken under ×8,000 magnification indicated that simvastatin ameliorates radiation-induced apoptosis in mice thymi (B). Red arrows indicate apoptotic cells. Values are means ± SEM. C, control; S, simvastatin; R, ionizing radiation; RS, IR + simvastatin; n=6, *P<0.05, **P<0.01 vs. C group; ##P<0.01 vs. R group (One-way ANOVA, Newman-Keuls multiple comparison test).