Cellular redoxbalance is maintained by various antioxidative systems among those is the thioredoxin system consisting of thioredoxin, thioredoxin reductase and NADPH. In the present study, we examined the effects of caloric restriction (CR) on the expression of the cytosolic and mitochondrial thioredoxin system in skeletal muscle and heart of senescent and young rats. Mitochondrial thioredoxin reductase (TrxR2) is significantly reduced in aging skeletal and cardiac muscle and renormalized after CR, while the cytosolic isoform remains unchanged. Thioredoxins (mitochondrial Trx2, cytosolic Trx1) are not influenced by CR. In skeletal and cardiac muscle of young rats, caloric restriction has no effect on the expression of thioredoxins or thioredoxin reductases. Oxidative stress in vitro (10–100mM H2O2 in C2C12 myoblasts) induces TrxR2 and Trx2 without alterations in the cytosolic isoforms. SiRNA-mediated reduction of TrxR2 in myoblasts under exposure to 20mM ceramide or 10ng/ml TNF-alpha causes enhancement of nucleosomal DNA-cleavage, caspase 9 activation and mitochondrial ROS release together with reduced cell viability, while this TrxR2 reduction is without effect in myoblasts under basal conditions. Taken together, aging is associated with a TrxR2 reduction in skeletal muscle and heart which enhances susceptibility to apoptotic stimuli but is renormalized after short-term CR. Exogenous oxidative stress is unable to mimic these age-related changes.