Malfunction of mitochondrial complex I caused by nuclear gene mutations causes early-onset neurodegenerative diseases. Previous work using cultured fibroblasts of complex-I-deficient patients revealed elevated levels of reactive oxygen species (ROS) and reductions in both total Ca(2+) content of the endoplasmic reticulum (ER(Ca)) and bradykinin(Bk)-induced increases in cytosolic and mitochondrial free Ca(2+) ([Ca(2+)](C); [Ca(2+)](M)) and ATP ([ATP](C); [ATP](M)) concentration. Here, we determined the mitochondrial membrane potential (Delta psi) in patient skin fibroblasts and show significant correlations with cellular ROS levels and ER(Ca), i.e., the less negative Delta psi, the higher these levels and the lower ER(Ca). Treatment with 6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox) normalized Delta psi and Bk-induced increases in [Ca(2+)](M) and [ATP](M). These effects were accompanied by an increase in ER(Ca) and Bk-induced increase in [Ca(2+)](C). Together, these results provide evidence for an integral role of increased ROS levels in complex I deficiency and point to the potential therapeutic value of antioxidant treatment.