Thioredoxin reductase (TR), a flavoprotein, catalyzes the reduction of oxidized thioredoxin in a NADPH-dependent manner, and contains a selenocysteine residue near the C-terminus. TR plays an important role in protecting against oxidative stress and in regulating cell growth and cell death. Constitutive TR expression has been observed in several cell types of the mammalian body, including endothelial cells. The latter are continually exposed to both exogenous and endogenous sources of nitric oxide (NO) and NO-derived species. Reactive nitrogen species (RNS) are associated with pathological events, contributing to the cell and tissue damage accompanying inflammation, atherogenesis and autoimmune diseases. In this study, we report on the effect of peroxynitrite on TR in human umbilical vein endothelial cells (HUVECs). Exposure to the peroxynitrite donor SIN-1 for 1 h resulted in a decrease in TR activity. Interestingly, the activity was completely restored within 24 h. To further examine this mechanism, the expression of TR at the mRNA and protein level was examined. TR mRNA levels were markedly increased by treatment of SIN-1 within 6 h, and TR protein level was also increased after the treatment in HUVECs. These results suggest that the inactivation of TR by peroxynitrite might be involved in the upregulation of the TR gene in HUVECs. Therefore, HUVECs have a unique protective mechanism that allows the maintenance of balance in intracellular redox status via TR induction as an adaptive response to nitrooxidative stress.