Abstract The neuropathological and clinical symptoms of Huntington's disease (HD) can be simulated in animal model with systemic administration of 3-nitropropionic acid (3-NP). Energy defects in HD could be ameliorated by administration of coenzyme Q 10 (CoQ 10), creatine, or nicotinamid. We studied the activity of creatine kinase (CK) and the function of mitochondrial respiratory chain in the brain of aged rats administered with 3-NP with and without previous application of antioxidants CoQ 10 + vitamin E. We used dynamic and steady-state methods of in vivo phosphorus magnetic resonance spectroscopy ( 31P MRS) for determination of the pseudo-first order rate constant ( k for) of the forward CK reaction, the phosphocreatine (PCr) to adenosinetriphosphate (ATP) ratio, intracellular pH i and Mg i 2+ content in the brain. The respiratory chain function of isolated mitochondria was assessed polarographically; the concentration of CoQ 10 and α-tocopherol by HPLC. We found significant elevation of k for in brains of 3-NP rats, reflecting increased rate of CK reaction in cytosol. The function of respiratory chain in the presence of succinate was severely diminished. The activity of cytochromeoxidase and mitochondrial concentration of CoQ 10 was unaltered; tissue content of CoQ 10 was decreased in 3-NP rats. Antioxidants CoQ 10 + vitamin E prevented increase of k for and the decrease of CoQ 10 content in brain tissue, but were ineffective to prevent the decline of respiratory chain function. We suppose that increased activity of CK system could be compensatory to decreased mitochondrial ATP production, and CoQ 10 + vitamin E could prevent the increase of k for after 3-NP treatment likely by activity of CoQ 10 outside the mitochondria. Results of our experiments contributed to elucidation of mechanism of beneficial effect of CoQ 10 administration in HD and showed that the rate constant of CK is a sensitive indicator of brain energy disorder reflecting therapeutic effect of drugs that could be used as a new in vivo biomarker of neurodegenerative diseases.