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The responses of the kidney to angiotensin depend on its dosage and on the state of sodium and water balance (1-4). T o eliminate extrarenal influences on renal hemodynamics and tubular function, the isolated kidney, perfused a t a constant pressure, was used to study the action of angiotensin on various parameters. I n former experiments we showed that renal blood flow ( R B F ) of the isolated rabbit kidney, perfused with an angiotensinogen-free medium, was constant for the total period of 90 min, whereas glomerular filtration rate (GFR) decreased slightly (5). When small amounts of rabbit angiotensinogen were added to the perfusion fluid, GFR and filtration fraction ( F F ) increased (6). Since this effect was not observed with renin-depleted kidneys taken from rabbits pretreated with DOCA and salt, it was assumed that endogenous renin, by releasing angiotensin, acted on the efferent arterioles and in this way affected GFR. To support this interpretation, we studied the action of exogenous angiotensin on GFR and FF in the isolated rabbit kidney.Materials and Methods. Rabbits of both sexes, weighing 2.5 to 3.5 kg, were anesthetized with pentobarbital (30-50 mg0kg-I iv) . Polyethylene catheters were placed into the ureter and into the artery of the left kidney. The left renal artery was connected to the perfusion system, and the blood was washed out with Tyrode's solution. Subsequently the kidney was transferred into a moist chamber. The perfusion fluid was free from angiotensinogen and consisted of Tyrode's solution containing 4 g of purified bovine albumin/ 100 ml ( Behrins-Werke, Marburg), to which washed rabbit erythrocytes had been added, resulting in a hematocrit of 40 ~0 1 % . The closed perfusion system had a total volume of 70 ml, and the perfusion was maintained by a peristaltic pump (Harvard Apparatus Mod. 1210). The perfusion fluid was oxygenized by a perspex disc oxygenator, with a gas mixture of 95% 0 2 and 5% COa. All tubes were of silicon material. The whole perfusion system and the moist chamber were kept a t a temperature of 38". With the aid of a Statham transducer (Mod. P 23 D b ) , perfusion pressure was recorded a t a tube connected to the cannula in the renal artery. During the total perfusion period, the pressure was kept constant by means of an electronic device regulating the perfusion volume. Variations in renal vascular resistance were compensated by changes in perfusion volume. Perfusion pressure was 97.1 _t 3.0 (SD) mm Hg.The venous effluent was collected in a siliconized funnel, from where it was directly returned to the oxygenator. The same was done with the urine. The perfusion volume was measured directly by a glass cylinder every 2 min. The urine volume was measured by a glass cylinder during the clearance periods. Urine samples taken for analyses were replaced by corresponding volumes of Tyrode's solution.GFR was determined by inulin clearance after inulin had been added to the perfusion fluid, resulting in a concentration of 0.5 mg.ml-2; the resorcinol method (7) was used for...

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