The use of the tests of kidney function has become so common a procedure in our hospitals in the past few years that little time need be wasted in recapitulating them. In general, four principles are involved: The determination of the rate of excretion in the urine of a known amount of a chemical substance, injected or ingested. The determination of the degree of retention in the blood of various normal metabolic products. The comparison in a patient on a known test diet of the ingestion and excretion of nitrogen, sodium chlorid, water, etc. A determination of the ratio between the concentration of various metabolic products, ordinarily urea, in the blood, and their excretion in the urine, the result being expressed as a ratio of excretion or coefficient. Of the first group, the procedure most widely used is the phenolsulphonephthalein test of Rowntree and Geraghty, which determines the amount of the dye excreted