Stimulation of the T lymphocyte antigen receptor‐CD3 complex (TCR‐CD3) causes T cell activation by a process associated with increased phosphatidylinositol‐specific phospholipase C (PI‐PLC) activity. Evidence exists suggesting that GTP‐binding (G) proteins, particularly the pertussis toxin (PT)‐sensitive Gi proteins, participate in this signal transduction pathway. To clarify the role of Gi proteins in TCR‐CD3 signaling, and to investigate other possible functions of Gi molecules in T cells, we expressed the S1 subunit of PT in the thymocytes of transgenic mice using the lymphocyte‐specific lck promoter. Transgenic thymocytes contained S1 activity and exhibited profound depletion of Gi protein PT substrates in a manner suggesting their inactivation by S1 in vivo. Nevertheless, treatment of transgenic thymocytes with mitogenic stimuli provoked normal increases in intracellular free Ca2+ concentrations and IL‐2 secretion, indicating that Gi proteins are not required for T cell activation. These normal signaling responses notwithstanding, mature thymocytes accumulated in lck‐PT mice and did not appear in secondary lymphoid organs or in the circulation. Viewed in the context of the known features of Bordetella pertussis infection, our results suggest that a PT‐sensitive signaling process, probably involving Gi proteins, regulates thymocyte emigration.