Distribution profiles of superoxide dismutase (SOD) isoenzymes in various tissues of domestic rabbits with VX-2 carcinoma in the maxillary sinus were compared with those of control rabbits. 1. Cu, Zn-SOD and Mn-SOD activities were highest in the liver and lowest in the submaxillary gland of tissues from control rabbits. 2. Cu, Zn-SOD activity in the liver of rabbits decreased significantly 3 weeks after the transplantation of VX-2 carcinoma in the maxillary sinus. Mn-SOD activity did not decrease significantly until 5 weeks after the transplantation. 3. In the VX-2 carcinoma Mn-SOD activity was not detected and Cu, Zn-SOD activity decreased remarkably compared with other tissues. These results suggest that VX-2 carcinoma lost most of the ability to defend against oxygen toxicity and this ability weakened only in the liver among tissues of rabbits bearing VX-2 carcinoma in the maxillary sinus.