The effects of idebenone on survival time of mice subjected to hypoxia induced by N2 and CO2 inhalation and KCN injection were studied. Idebenone (10, 20, 50 and 100 mg/kg, i.p.) prolonged the survival time of mice exposed to a hypoxic condition (98% N2, 2% O2) in a dose-dependent manner: significant prolongations were observed at doses higher than 20 mg/kg. The drug also exerted a similar elongation effect at the same doses under conditions of 100% CO2 gas inhalation and KCN injection (3 mg/kg, i.v.). The results suggest that idebenone protects against hypoxia by improving cerebral energy metabolism.