When roots of tomato plants are infected withFusarium, systemic changes are induced in the activities of glycolic acid oxidase and glyoxylic reductase of the leaves. A marked decrease in glycolic acid oxidase activity is apparent 8–16 days after inoculation when the leaves show chlorotic symptoms. The depressed activity of this enzyme is due to a general decrease in the concentration of its flavin coenzyme, FMN. Both FMN and FAD begin to decrease after 8 days. On the other hand glyoxylic acid reductase shows an increased activity 20 days after infection. These alterations in enzymic activity result in a twofold accumulation of glycolic acid 20 days after infection even though the fungus is never present in the leaves.