Thymic T cell anergy, as manifested by thymocyte proliferative unresponsiveness to antigens expressed in the thymic environment, is commonly believed to mediate the acquisition of immunological self-tolerance. However, we previously found that thymic T cell anergy may lead to the breakdown of tolerance and predispose to autoimmunity in nonobese diabetic (NOD) mice. Here, we show that NOD thymic T cell anergy, as revealed by proliferative unresponsiveness in vitro after stimulation through the T cell receptor (TCR), is associated with defective TCRmediated signal transduction along the PKC/p2rasp42mapk pathway of T cell activation. PKC activity is reduced in NOD thymocytes. Activation of p21ras is deficient in quiescent and stimulated NOD T cells, and this is correlated with a significant reduction in the tyrosine phosphorylation of p42mapk, a serine/threonine kinase active downstream of p21ras. Treatment of NOD T cells with a phorbol ester not only enhances their p21ras activity and p42mapk tyrosine phosphorylation but also restores their proliferative responsiveness. Since p4mapk activity is required for progression through to S phase of the cell cycle, our data suggest that reduced tyrosine phosphorylation of p42mapk in stimulated NOD T cells may abrogate its activity and elicit the proliferative unresponsiveness of these cells.