The relative significance of reduced excretion of urinary constituents and reduced renal mass, as stimuli to growth of one kidney after the other has been removed, has been investigated. 2. To abolish the excretory function of one kidney without removing it, the right ureter was drained into the vena cava through a compound cannula for 6 weeks. Uretero-caval anastomoses were performed in twenty-four male rats at 10 weeks of age: six survived without evidence of ureteric obstruction (and a further five with minimal obstruction). 3. The rats with anastomoses grew less than six other rats from which the right kidney had been removed or six which had been submitted to a sham operation, and their plasma urea and creatinine concentrations were higher. 4. Relative to body weight, the dry weight of each kidney after uretero-caval anastomosis without obstruction was 18% greater than after sham operation; taking both kidneys together, the total increase was almost as much as in the left kidney alone after right nephrectomy (46%). 5. Histologically and in terms of DNA concentration, the growth of both kidneys after uretero-caval anastomosis was of the same kind as in the left kidney after right nephrectomy. 6. The return of urine from one kidney into the blood provided a powerful stimulus to renal growth in spite of the restraining effect of increased renal mass.
