Relative concentration of chloride, bicarbonate, phosphate, urea and uric acid in blood and in bladder urine was determined in Rana catesbiana under normal conditions. Concentration in the blood of each of these substances in turn was then increased. Resultant change in concentration of the substance in the urine was then studied. Chloride of the blood, if increased to the point of toxicity, raises chloride concentration in the urine to a point slightly higher than that of the blood. Simultaneously, determinations of carbonates showed that molecular concentration of the 2 salts was never higher in the urine than in the blood. Phosphates, normally present in the urine in small amounts, were concentrated as much as 6-fold. Toxicity limits further concentration. Urea is normally more concentrated in the frog''s urine than in the blood. Concentration of urea was increased to as much as 74 times the normal value. Phenolsulphonephthalein was excreted in the same manner as urea. It is thus seen that while urea and phenolsulphonephthalein can be efficiently concentrated, phosphates are less easily so, chloride and bicarbonate not at all. Concentration of urea in the kidney is thought to be due to an accumulation of that substance in the cells.