Tryptophan catabolism is a tolerogenic effector system in regulatory T cell function, yet the general mechanisms whereby tryptophan catabolism affects T cell responses remain unclear. We provide evidence that the short-term, combined effects of tryptophan deprivation and tryptophan catabolites result in GCN2 kinase-dependent down-regulation of the TCR ζ-chain in murine CD8⁺ T cells. TCR ζ down-regulation can be demonstrated in vivo and is associated with an impaired cytotoxic effector function in vitro. The longer-term effects of tryptophan catabolism include the emergence of a regulatory phenotype in naive CD4⁺CD25⁻ T cells via TGF-β induction of the forkhead transcription factor Foxp3. Such converted cells appear to be CD25⁺, CD69⁻, CD45RB^low, CD62L⁺, CTLA-4⁺, BTLA^low and GITR⁺, and are capable of effective control of diabetogenic T cells when transferred in vivo. Thus, both tryptophan starvation and tryptophan catabolites contribute to establishing a regulatory environment affecting CD8⁺ as well as CD4⁺ T cell function, and not only is tryptophan catabolism an effector mechanism of tolerance, but it also results in GCN2-dependent generation of autoimmune-preventive regulatory T cells.