Abstract 1. 1. The concentration of the free radicals in muscle tissue mitochondria changes with the phase of muscle contraction. 2. 2. Potassium cyanide (KCN) influences the shape of the isotonic tetani mechanograms; the contraction is still strong and qualitatively comparable in amplitude to untreated controls. 3. 3. On the other hand, the correlation between the free radical concentration and muscle contraction is lost. 4. 4. The free radical concentration in isolated mitochondria is influenced by KCN and potassium ferricyanide. 5. 5. The free radical concentration changes due to the electron flow in the respiratory chain. 6. 6. Inhibition of the respiratory phosphorylation by KCN could affect the free radical level.