This study was performed by the sister chromatid exchanges (SCEs) and micronuclei (MN) assays to investigate the adaptive response to ultraviolet light (UV) or ethyl methanesulfonate (EMS) in Chinese hamster ovary (CHO) and Sarcoma 180 (S180) cells. The pretreatment with 1 J/m$^2$ UV or 2 mM EMS decreased the frequency of SCEs induced by the treatment with 5 J/m$^2$ UV or 8 mM EMS in CHO cells. The pretreatment with UV (1 or 2 J/m$^2$) or EMS (1, 2 or 3 mM) did not affect the SCEs induced by the treatment with 7 J/m$^2$ UV or 10 mM EMS in S180 cells. On the other hand, the pretreatment with 1 J/m$^2$ UV or 2 mM EMS decreased the frequency of MN induced by the treatment with 5 J/m$^2$ UV or 8 mM EMS in CHO cells. The pretreatment with UV (1 or 2 J/m$^2$) or EMS (1, 2 or 3 mM) did not affect the frequency of MN induced by the treatment with 7 J/m$^2$ UV or 10 mM EMS in S180 cells. It is suggested that there are adaptive responses at the level of chromosome and micronuclei to UV and EMS in CHO cells.