Catalase activity was followed up in the hearts and livers of CD 1 mice treated with Doxorubicin 4 mg/Kg, i.v., weekly for 9 weeks. In this murine model the antiblastic induces cardiac morphological lesions which are progressively severer with the increase of the administered cumulative dose. Heart catalase showed a consistent elevation which reached a maximum (+116.2%, P less than 0.05) after the 5th dose. In the case of hepatic catalase no significant variation was observed except a transitory elevation following the first administration. The specific increase of heart catalase activity following multiple Doxorubicin doses could be an indicator that an enhanced free radical generation acts "in vivo" along with the onset of the cardiac lesions due to antiblastic.