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By a specially devised technique, indwelling catheters have been placed in the portal vein and abdominal aorta of rats, permitting pressure measurements and sampling in unanaesthetized animals. A cuff was placed around the portal vein; inside the cuff was a balloon which could be reversibly inflated for any desired period from outside, so as to occlude the portal vein completely. Pressures in portal vein and aorta, together with urine flow rate determined with the aid of a bladder catheter, were recorded simultaneously in unanaesthetized animals. Portal occlusion for less than 20 sec causes temporary anuria (30-60 sec) but no other effect. Occlusion for 20-30 sec causes a small and fleeting decline of blood pressure which is followed by submaximal antidiuresis. With more prolonged occlusion the fall of blood pressure is directly related to the duration of occlusion, but the duration and degree of the resulting antidiuresis remains the same. A second occlusion during the height of antidiuresis is without effect on the duration of the antidiuresis. From the shape of the curve relating portal pressure during occlusion to time, contraction of the mesenteric veins can be inferred. Blood samples taken during the antidiuresis after occlusion exert an antidiuretic effect when injected into a test rat. The three effects produced by portal occlusion-contraction of the mesenteric veins, reduction of renal blood flow (indicated by anuria), and antidiuresis (produced by release of ADH) all act to conserve blood volume. (Author)