Background: Whether mitochondrial Ca 2ϩ extrusion is mediated by NCLX (mitochondrial sodium/calcium exchanger) or LETM1 (leucine zipper-EF-hand-containing transmembrane protein 1) and controls matrix redox state is unknown. Results: NCLX, but not LETM1, increases Ca 2ϩ extrusion, limits NAD(P)H production, and promotes matrix oxidation. Conclusion: NCLX controls the duration of matrix Ca 2ϩ elevations and their impact on redox signaling. Significance: NCLX is a potential target for the treatment of redox-dependent diseases.