T cell anergy has been correlated with defective Ras signaling. However, neither a causal relationship nor the mechanism of Ras hypoactivation have been established. Using adenoviral transduction of CAR Tg T cells to enable genetic manipulation in nonproliferating cells, we show that Ras61L restores IL-2 production and MAP kinase signaling in T cells anergized in vitro or in vivo. A gene array screen revealed upregulated diacylglycerol kinase (DGK) in the anergic state, which was confirmed by RT-PCR and Western blot analysis. A DGK inhibitor significantly restored IL-2 production by anergic cells. Our data support a causal role for DGK and defective Ras signaling in T cell anergy.