According to Maekawa, the "true cause" of hypertension is considered to be disturbances of the ATP-ATPase system, and especially release of ATPase of the kidney into blood stream.Here, a precise study on the pressor effect of the kidney-ATPase and at the same time, its effects on renal hemodynamics and other renal functions were studied. Then it was studied to produce hypertension in dogs similar to self-sustaining post-DCA-hypertension of rats by administration of the dialyzed kidney-ATPase.And the effects of ATP on blood pressure and on renal hemodynamics in dogs were observed, too.1) Acute experiment : a) A female dog was anesthetized with iso-mytal sodium ; mean blood pressure was 88-120 mm.Hg before injection. Upon intravenous injection of 60cc of dialyzed kidney-ATPase, pressure rised to 130-155 mm.Hg at the end of injection. Further intravenous injection of 45cc. led to more rise of pressure (140-160mmHg).b) Next, it was attempted to produce the typical pattern of acute malignant hypertension in dog. The same dog was used. With drip infusion of dialyzed kidney-ATPase blood pressure rised gradually to 120→137→170mmHg, and hematuria, albuminuria and glycosuria occurred. The elevation of blood pressure as well as changes of urine became more and more intense with injection. The pattern is undoubtedly that of the acute malignant hypertension.2) Relative chronic experiment : 4 dogs were used, and were daily injected intravenously with 15∼90cc. for 8∼25days. The animals developed a facilitation to the repeated injection of kidney-ATPase, although expected result could not be obtained on the account of lack in time.3) Chronic experiment : 4 dogs, including 2 unilaterally nephrectomized animals, were used. One nephrectomized and one normal dogs were daily injected intravenously with 30∼50cc. of dialyzed kidney-ATPase and other two additionally with 50∼200mg. of cholesterol intravenously.The duration of the experiment was 2∼4 months. During this experiment NaCl load (5∼15gm.) was added.Two dogs without cholesterol load showed a rise of mean pressure by 10∼20mm.Hg, sustaining this level for 2∼3 weeks after withdrawal of the administration of kidney-ATPase. And blood pressure was fluctuated.In the other two treated with ATPase and cholesterol, a significant elevation of blood pressure was not observed.Changes of RPF, GRR and FF were not consistent, but apt to increase in general. This corresponds with the early stage of essential hypertension of man.4) During injection of kidney-ATPase, RPF decreased by ca. 40%, FF rised slightly and urine flow decreased markedly.5) Furthermore, above-mentioned crude "Kidney-ATPase" was divided into two fractions by absorption procedure (Maekawa & Nakajima), i. e. pressor fraction "C-K" and depressor fraction "M".Then, effects of the two fractions on renal tubular activity were observed. By intravenous injection of pressor fraction "C-K", TmPAH decreased by 20%, TmG decreased by 70%, GFR decreased by 83% and urine flow decreased markedly, too.By intravenous injection of depressor fraction "M", GFR and urine flow decreased moderately, TmPAH increased slightly and TmG decreased by 61%.6) By intravenous injection of adenosine triphosphate, RPF and urine flow increased and renal vascular resistance decreased, contrary to ATPase. And TmPAH and TmG increased markedly. Na clearance and pottasium clearance increased too.