Several recent studies have demonstrated that thioredoxin (Trx) is an important antiapoptotic/cytoprotective molecule. The present study was designed to determine whether Trx activity is altered in the aging heart in a way that may contribute to increased susceptibility to myocardial ischemia/reperfusion (MI/R). Compared to young animals, MI/R-induced cardiomyocyte apoptosis and infarct size were increased in aging animals (p<0.01). Trx activity was decreased in the aging heart before MI/R, and this difference was further amplified after MI/R. Trx expression was moderately increased and Trx nitration, a posttranslational modification that inhibits Trx activity, was increased in the aging heart. Moreover, Trx-aptosis-regulating kinase-1 (Trx-ASK1) complex formation was reduced and activity of p38 mitogen-activated protein kinase (MAPK) was increased. Treatment with FP15 (a peroxynitrite decomposition catalyst) reduced Trx nitration, increased Trx activity, restored Trx-ASK1 interaction, reduced P38 MAPK activity, attenuated caspase 3 activation, and reduced infarct size in aging animals (p<0.01). Our results demonstrated that Trx activity is decreased in the aging heart by posttranslational nitrative modification. Interventions that restore Trx activity in the aging heart may be novel therapies to attenuate MI/R injury in aging patients.