Simian virus 40 T-tumor antigen (SV40 T-ag) can induce a wide variety of tumors in hamsters and neonatal mice. These tumorigenic effects are predominantly due to the activity of early viral gene products, large T-antigen and small t-antigen. We have analyzed the expression of a DNA repair gene, N-methylpurine-DNA glycosylase (MPG), from different tissues of a non-transgenic (control) and SV40 T-ag expressing transgenic mice at the mRNA level. Expression of the transgene in thymus of adult mice was also detected by the presence of SV40 T-ag mRNA. Non-transgenic mice did not express the SV40 T-ag gene in their thymus, while the mRNA for MPG was found in thymus from both of transgenic and non-transgenic mice. The MPG gene was expressed in various tissues and is regulated in a tissue-specific manner. Northern blot analysis revealed that the transgenic mice showed considerably higher expression of MPG in the thymic carcinomas. The level of MPG mRNA in the thymic carcinoma was elevated about 5.7 fold, as compared with those found in the control thymus. MPG expression was significantly increased, either directly or indirectly, by the SV40 T-ag gene product. These findings provide the first in vivo observations that the SV40 T-ag gene induced thymic carcinomas associated with the activation of the DNA repair gene, MPG.