We investigated the response to oxidative stress in a model system established in C2C12 cells stably transfected with myotonin protein kinase (MtPK) cDNAs having 5, 46, 60, or 160 CTG repeats. The transformants showed CTG repeat number-dependent susceptibility to oxidative stress. Mutant MtPK cDNA transformants containing 160 CTG repeats showed apoptotic cell death by the exposure to an oxidant, a very low level of methylmercury. The addition of the antioxidant Trolox protected transformants against apoptosis. Oxidative stress activated the extracellular signal-regulated kinases (ERKs) pathway leading to cell survival in mild-type MtPK cDNA transformants, whereas mutant MtPK cDNA transformants having 160 CTG repeats were defective in the induction of the ERK pathway, although the activation of stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK) was strong and sustained. These results suggest that the susceptibility to oxidative stress in mutant MtPK cDNA transformants involves differential signaling pathways evoked following oxidative stress.