Fas-mediated apoptosis is a form of cell death that operates through a receptor-ligand interaction. The FasR has been implicated directly in peripheral T cell tolerance and activation-induced apoptosis of T cells in vitro, although to date its expression on murine peripheral T cells has been characterized incompletely. In this study, we document substantial expression of FasR on the vast majority of recent thymic emigrants and resting peripheral T lymphocytes. FasR ligation can induce death in a minor (approximately 5%) subset of these cells. By contrast to rather slow activation-mediated FasR up-regulation in vitro, we demonstrate that in vivo T cell activation by alpha CD3 mAb or superantigen results in rapid up-regulation of the FasR. This up-regulation is paralleled by the kinetics of activation-induced apoptosis in lymph node T cells. However, we demonstrate that the FasR is not necessary for activation-induced cell death. Lymph node T cells from young, healthy, FasR expression-deficient MRL-Ipr/Ipr and animals could be activated in vivo through the TCR-CD3 complex. Most importantly, MRL-Ipr/Ipr T cells underwent massive activation-induced apoptosis in response to high and intermediate doses of alpha CD3. At a low alpha CD3 dose, however, both MRL-Ipr/Ipr and MRL +/+ T cells were activated similarly, but only the latter underwent adequate apoptosis. Taken together, these findings suggest that in vivo, the Fas pathway may not be the only regulator of activation-induced T cell death, but that this pathway may be critical in regulating responses to weak stimuli.