The purpose of this study is to compare the oxidase activity of the healthy paulownia tree with that of the diseased tree affected by witche's-broom, and to find some explanations on the abnormal metabolism of the disease tree. The oxidase activity was determined by the Warburg micro-manometric method. The results are as follows: 1) The enzyme solution from paulownia leaf is not responsible for the oxidation of resorsin and oxalic acid but is fairly capable of catalyzing the oxidation of hydrochinon and ascorbic acid; and it is able to oxidize pyrogallol seventeen times and catechol twenty six times stronger than to oxidize hydrochinon. The oxidases of the diseased and the healthy leaves show the same tendency against those substrates. 2) Among the oxidase activities of leaves, shoots, ad roots, it is found that the strongest is that of leaves. The oxidase activity of shoots is about ten per cent of that of leaves, and that of roots is about four per cent. These tendencies are the same both in the case of the healthy and the diseased leaves. The oxidase activity of the diseased leaves is twenty per cent greater than that of the healthy ones. The oxidase activity of the diseased shoots is scarcely different from the healthy ones. The oxidase activity of the diseased roots is forty percent greater than that of the healthy. 3) Observations of the seasonal changes of oxidase activity show that it is greater in the diseased leaves than in the healthy ones at the rate of 29% in July, 38% in August, 40% in September and 20% in October. The increase of polyphenol oxidase activity and respiration in the diseased leaves, indicate that the polyphenol oxidase may play a part of the respiration as a terminal oxidase of paulownia tree. It may be partly explained by the increase of polyphenol oxidase activity in the diseased leaves that the sap of the diseased leaves changes to brown more quickly than that of the healthy one.