Abstract: In or der to understand the biological effects of env ironmental chemical and physical factors on the organism,the influence of methtylmercury and ionizing radiation of different dose levels on adaptation and DNA sythesis of thymus of mice was studied by means of ³H-TdRincorporation method.The results showed that meththylmer cury could inhibit DNA synthesis of thy mocytes.Low dose radiation(0.075 Gy)could stimulate DNA synthesis of thymocytesyhe amount of ³H incorporation was higher than that of control.High dose radiation(2Gy)could inhibit DNA synthesis of thymocytes.The deg ree of DNA damagewas obviously decreased and amount of ³H incorporation was increased in LDR(0.075Gy) pretreatment group.The degree of this effect in HDR(2Gy)plus Me Hg was significantly lower than MeHg.It indicated that LDR could induce Hormesis,and protect from DNAdamage.The relative mechanism could be further discussed.