1. A new test for measuring the ability of the kidney to concentrate the urine is described. The test is based on the administration of a diuretic followed by complete food and fluid deprivation for 16 h. Slow-release preparations of sodium and potassium are used to replace the loss due to the diuretic.

2. Values reached with the proposed test are not as high as those after the Pitressin-dehydration test, but the differences are small and not statistically significant.

3. The proposed test avoids the disadvantages of prolonged fluid deprivation or an injection of 5 units of Pitressin tannate in oil.

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