1) Using the mitochondria treated with acetone, measurement was made about the activity-restoring effect of ubiquinone (35)(I) and related compounds on the succinate oxidase system, and as the partial structure required for the effect the following points were made clear : i) the quinone nucleus is necessary, ii) the methoxyl at position of 3 may be changed to a hydroxyl but if it is converted to acetoxyl the effect is lost, iii) the length of the isoprene side chain has no influence so long as the number of the isoprene is 7∼10, but when the double bonds of the side chain are saturated, the activity decreases to 1/2, and when one of the double bonds is conjugated with the quinone nucleus, the activity lowers to 1/3. 2) When added to beef-heart mitochondria, some substances in Table IV other than ubiquinone (35)(I) and ubichromenol (35) acetate (IX) show slight inhibition of the succinate oxidase system.