The pharmacological action of phenol-glycuronic acid, urochloralic acid, glycuronic acid and benzoyl-glycuronic acid have been investigated in our Laboratory by Ishii, Matsushima, Tanaka and Sakata. Following the line of these investigations the author has studied the pharmacological action of ornithuric acid prepared from the urine of hen by giving sodium benzoate. The results obtained are as follows: (1) Ornithuric acid when given to the rabbit in its general circulation gives rise to a marked diuresis. Meanwhile it causes practically no change in general blood pressure, nor in the water content of the blood, and only the oncometric record shows an augmentation of kidney volume in every case. And in denervated or isolated kidney an increased perfusion rate is obtained. So it seems that in this vasomotor reaction the central nervous mechanism plays no part. (2) Ornithuric acid exerts diuretic action also on cold blooded animal. Analysis effected on perfused toad kidney showed that it concerns in this diuresis the glomerular system only, and that tubular system has nothing to do with. (3) The action of ornithuric acid on the blood vessel of the organs other than kidney is as follows: - Hind leg vascular system of the frog is dilated by low and constricted by high concentration of this substance in the perfusing fluid. Vascular system of the rabbit's ear shows similar reaction toward this substance. But coronary vascular system is constricted notwithstanding the concentration of this substance in the perfusing fluid. (4) Isolated hearts of toad and rabbit augment their tone and diminish the amplitude of their contraction by middle dosage and especially in the beginning, but then tone returns gradually to normal, accompanying an increase in the amplitude of contraction. By larger dosages there occurs an increase in amplitude of the heart beat during only a very short period at the beginning, then the amplitude decreases progressively to be finally arrested at systole. It must be noted that in every case there is no change in the frequency of heart beat. (5) In isolated smooth muscular organs of the rabbit, such as intestine, uterus and bladder, an augmentation in the tone and the spontaneous movement are observed for an appropriate dosage. Atropine has no influence on this effect, also the isolated trigonal and basal parts of the rabbit's bladder have shown no difference in their reaction toward this substance. (6) From these upper three results it may be concluded that ornithuric acid does not exert its action by way of the nervous mechanism but directly on the smooth muscular system.