To evaluate the in vivo effect of immunosuppressive glucocorticoids on CD4(+)CD25(+) T regulatory cells, we injected dexamethasone (Dex) into BALB/c mice. Administration of Dex enhanced the proportion of CD4(+)CD25(+) cells and the ratio of CD4(+)CD25(+) cells to CD4(+)CD25(-) cells in the lymphoid organs, especially in the thymus. This correlates with our in vitro observation that CD4(+)CD25(+) T cells express higher levels of glucocorticoid receptor and Bcl-2, and are therefore more resistant to Dex-mediated cell death than CD4(+)CD25(-) T cells. Furthermore, IL-2 selectively protected CD4(+)CD25(-) T cells from Dex-induced cell death, while IL-7 and IL-15 did not exert preferential protective effects. Dex-treated CD4(+)CD25(+) T cells expressed higher levels of intracellular CTLA-4 and surface glucocorticoid-induced TNF receptor than fresh CD4(+)CD25(+) T cells, but still failed to respond to TCR stimulation and inhibited proliferation of CD4(+)CD25(-) T cells. These results suggest that, in addition to suppressing cytokine transcription, Dex treatment is permissive for the survival of functional CD4(+)CD25(-) T regulatory cells, and this property may contribute to the anti-inflammatory and immunosuppressive efficacy of glucocorticoids. Our data also suggest that selective protection of CD4(+)CD25(+) T cell from apoptosis may constitute a role in immune tolerance for IL-2.