1. XA was demonstrated on paper-chromatography in the urine of Graves' disease patients who had been administered 1.0gm of tryptophan, while no XA was detected in the urine of healthy subjects. 2. The blood sugar level of thyroxine rats was elevated following the production of XA due to the administration of tryptophan, while that of normal rats revealed no change. 3. Kynureninase activities of the liver of thyroxine rats showed a marked decrease while kynurenine-transaminase activities remained unchanged.