A gender difference in D-aspartate oxidase activity was demonstrated for the first time in rat tissues. 2. In adult Wistar rats, the hepatic enzyme activity was constantly higher in females than males. The renal activity showed a similar tendency. 3. The gender difference in the hepatic activity was not observed in newborn animals, but became apparent with growth almost in parallel with sexual maturation. 4. The hepatic activity in females was greatly affected by pregnancy and parturition.