1. In order to clarify further the phenomenon of post-obstructive diuresis, clearance and micro-puncture experiments were done before and after relief of partial ureteral obstruction in rats with a solitary hydronephrotic kidney.

2. Glomerular filtration rate, urine flow and sodium excretion increased markedly, whereas surface nephron glomerular filtration rate increased only slightly and intratubular pressure, proximal and distal tubular water reabsorption did not change significantly. Decreased tubular reabsorption in deeper nephrons and collecting ducts appeared to be of major importance in the post-obstructive diuresis after relief of chronic obstruction.

3. In order to examine further the distinctive functional characteristics of the chronically hydronephrotic kidney, the results were compared with control rats having a solitary normal kidney or a solitary remnant kidney with an intact renal medulla. Urine flow rate and sodium excretion were higher and urine osmolality was lower (P < 0.01) in post-obstructive kidneys when compared with either control group. There were no differences in glomerular filtration rate or surface nephron function which could account for the greater diuresis and natriuresis from the hydronephrotic kidney, thus confirming the importance of an abnormality in deep nephron or medullary function in post-obstructive diuresis.

4. There was a greater diuresis in post-obstructive rats with a marked increase in blood urea concentration. Water reabsorption in the distal nephron was decreased in such animals, as well as in urea-loaded rats with a remnant kidney, indicating the probable mechanism by which urea diuresis potentiates the phenomenon of post-obstructive diuresis.

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