Development of Th1 and Th2 effector lymphocytes is driven primarily by IL‐12 or IL‐4, but is also influenced by the strength of antigenic stimulation. However, the mechanism by which TCR signaling contributes to T cell polarization remains elusive. We show that in the presence of IL‐12 a short TCR stimulation can lead to efficient Th1 polarization and IL‐12 exerts its effect when present during, as well as after, TCR signaling. In contrast, Th2 polarization requires a prolonged TCR stimulation and IL‐4 is effective only when present during the period of TCR triggering. The simultaneous stimulation by TCR and IL‐4 is required to induce demethylation of IL‐4 and IL‐13 genes that accompanies the stochastic generation of Th2 cells producing either or both cytokines. Thus, the duration of TCR stimulation represents a crucial parameter that influences the response to polarizing cytokines and the acquisition of T cell effector functions.