To clarify the relationship between plasma antioxidant activity and diseases in dogs, plasma samples were collected from 6 healthy dogs and 16 diseased dogs (6 dogs with cancer, 5 dogs with hepatic disease, and 5 dogs with inflammation ), and measured superoxide anion scavenging activities. Antioxidant activities of canine plasma were evaluated by measuring their superoxide anion (O₂^-·) scavenging activities with electron spin response spectroscopy combined with spin trapping reagent, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Total O₂^-· scavenging activities in the presence of plasma of diseased dogs tended to be higher than those in healthy controls, especially significant higher activities in the presence of canine plasma of hepatic disease and inflammation were observed. In diseased dogs, KCN-insensitive activities, suggesting the activity of manganese-containing superoxide dismutase (Mn-SOD), were significantly higher than those in healthy controls. Therefore, it seems that there is a possibility of utilizing of plasma O₂^-· scavenging activity as one of clinical indicators for oxidative-related diseases such as cancer, hepatic disease and inflammation in dogs.