In this study, superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activities and malondialdehyde (MDA) levels were measured in heart tissues from guinea pigs treated with gentamicin and gentamicin plus vitamin E combination. Mean values were compared with those of the controls treated with only physiological saline solution. The activities of SOD and GSH-Px were found to be lower and the MDA level higher in the hearts from gentamicin-treated animals compared with those of the controls. In the gentamicin plus vitamin E group, however, tissue SOD activity was found to be increased and MDA level decreased significantly relative to the gentamicin group. GSH-Px activity was lowest in this group.