1. The excretion of urea and of chlorides in the normal individual is carried out according to definite laws, capable of numerical expression.
2. The rate of excretion of urea and of sodium chloride is determined by the concentration in the blood, the rate of water output, and indirectly by the weight of the individual.
3. The threshold of sodium chloride excretion is practically constant at about 5.62 grams of sodium chloride per liter of plasma. Slight variations in the threshold occur in normal individuals.
4. The rate of excretion of urea under the conditions found at any time can be measured directly in terms of the normal by an index of urea excretion.