Heterotrimeric Gi signaling regulates immune homeostasis, since autoimmunity occurs upon disruption of this pathway. However, the role of the lymphocyte-expressed Galphai subunits (Galphai2 and 3) on T cell activation and cytokine production is poorly understood. To examine this role, we studied T lymphocytes from mice deficient in the Galphai2 or Galphai3 subunits. Galphai2(-/-) but not Galphai3(-/-) splenocytes were hyper-responsive for IFN-gamma and IL-4 production following activation through the TCR. Galphai2(-/-) T cells had a relaxed costimulatory requirement for IL-2 secretion and proliferation compared to wild-type cells. Purified naïve Galphai2(-/-) T cells produced more IL-2 than naïve wild-type T cells following TCR activation, indicating that the hyper-responsive cytokine profile was not due to the expanded Galphai2(-/-) memory T cells, but involved an intrinsic T cell alteration. Cytokine hyper-responsiveness was not seen when purified Galphai2(-/-) T cells were stimulated with phorbol myristic acetate/ionomycin, localizing the alteration to a proximal TCR-specific signaling pathway. Galphai2(-/-) CD4(+) T cells were distinguished from wild-type or Galphai3(-/-) T cells by a globally augmented TCR-induced calcium response. These findings indicate that Galphai2(-/-) mice have an intrinsic CD4(+) T cell abnormality in TCR signaling which may be one cause of augmented T cell effector function and Galphai2(-/-) autoimmune susceptibility.