Objective: To analyze the effect of homeopathic Arnica on mitochondrial oxidative stress induced by Ca2+ plus inorganic phosphate and/or Fe2+-citrate-mediated lipid peroxidation through changes in oxygen consumption rates. Methods: Mitochondria were isolated by differential centrifugation from the livers of adult male Wistar rats which had been treated with Arnica montana 6cH, 12cH, 30cH or succussed 30% ethanol (control) for 21 days. Results: In the presence of antimycin-A, electron transport chain inhibitor, as evidenced by antimycin-A insensitive O2 consumption, Arnica inhibited lipid peroxidation of mitochondrial membranes. In oxidative stress conditions, in the presence of Ca2+ and inorganic phosphate, animals receiving Arnica 30cH had a significant decrease in mitochondrial O2 consumption compared to control animals. Conclusion: When administrated orally, Arnica 30cH protects against hepatic mitochondrial membrane permeabilization induced by Ca2+ and/or Fe2+-citrate-mediated lipid peroxidation and fragmentation of proteins due to the attack by reactive oxygen species.