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To evaluate the in vivo effect of immunosuppressive glucocorticoids on CD4⁺CD25⁺ T regulatory cells, we injected dexamethasone (Dex) into BALB/c mice. Administrationof 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 thethymus. 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 didnot 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 …