In cancer, the phenotype and/or the function of T cells may differ according to their distribution through immune-associated tissues, namely immune compartments. Here, in N-methyl-N-nitrosourea (MNU)-induced mammary carcinomas of rat as a relevant model for human breast tumors, the impact of tumor burden on the T cell subsets populating the tumor microenvironment, the tumor-adjacent and -opposite mammary lymph nodes, and the spleen was assessed. In the tumors, ratio of CD8+ cytotoxic and CD4+ helper T cells were not significantly different than other immune compartments. On the other hand, most of these cells were further identified with CD4+ CD25hi or CD4+ Foxp3+, CD8+ Foxp3+ regulatory phenotype. The selective presence of Tregs in the mammary tumors but not in neighboring-mammary tissue was also confirmed by the expression of Treg-associated genes. The percentage of CD161+ NKT cells was also significantly increased especially in the tumors and mammary lymph nodes. In the lymph nodes of tumor-bearing animals, in contrast to the spleen, total amount of CD8+ cells and CD4+ cells were increased but both of these compartments harbored high numbers of CD4+ CD25hi Treg cells. TGF-beta was determined as the major suppressive cytokine secreted by the immune cells of tumor-bearing animals, in addition, proliferation capacity of the T cells was diminished. Hence, the differential distribution of T cell subsets through the spleen, the mammary lymph nodes and the tumor mass in MNU-induced mammary tumor-bearing animals may contribute to a tumor-associated immunosuppression.