Thioredoxin-2 (Trx2) is a multifunctional, mitochondria-specific protein, which inhibits cell death. The mitochondrial permeability transition (MPT) is a distinct mechanism for cell death activated by oxidants and linked to both necrotic and apoptotic morphologies. We studied mitochondria from Trx2 transgenic mice to determine whether Trx2 protects against oxidant-induced MPT. All experiments were performed in isolated mitochondria. Results showed that Trx2 protected against MPT induced by exogenously added peroxide. Unexpectedly, Trx2 also protected against the MPT induced by Ca²⁺ in the absence of added peroxide. The results indicate that in addition to protecting against oxidative stress, Trx2 is an endogenous regulator of the MPT.