Regulatory T cells (T_reg cells) can express the transcription factors T-bet and GATA-3, but the function of this expression and whether such cells represent stable subsets is still unknown. By using various reporter tools, we found that the expression of T-bet and GATA-3 in T_reg cells was dynamically influenced by the cytokine environment. T_reg cell–specific deletion of the gene encoding either T-bet (Tbx21) or GATA-3 (Gata3) alone did not result in loss of T_reg cell function; however, mice with combined deficiency in both genes in T_reg cells developed severe autoimmune-like diseases. Loss of T_reg cell function correlated with upregulation of expression of the transcription factor RORγt and reduced expression of the transcription factor Foxp3. Thus, in the steady state, activated T_reg cells transiently upregulated either T-bet or GATA-3 to maintain T cell homeostasis.