The kidney effects the removal of solutes from the blood in a variety of ways: Ions, such as sulfate, phosphate, ethyl sulfate, calcium, magnesium and certain sulfonic acid dyes (Yoshida, '24; Hober, '30 a) are passively filtered through the glomeruli without any considerable subsequent dilution or concentration when later these substances pass along the tubules. Other substances of relatively small molecular volume such as the non-electrolytes, ethylene glycol, glycerol, acetamide and thiourea (Schmengler and Hober, '33) reach the fluid contents of the tubules not only by virtue of glomerular filtration, but also by diffusion through the tubular walls. A second class of substances, represented by chlorides (Bainbridge, Menzies and Collins, '14), amino acids (Robbins and Wilhelm, '33) and monosaccharides (Hamburger and Brinkman, '18 ; Clark, '22 ; Wearn and Richards, '24 ; Hausler, '31 ; Rober, '33), are also filtered through the glomeruli, but transported back into the blood by active work of the tubular epithelia. I n general these substances may be characterized as being useful to the organism. Representative of a third class are certain dyes like phenol red (Marshall and Crane, '24 ; Scheminzky, '29 ; Liang, '29 ; Hober and Meirowsky, '32 ; Chambers and Cameron, '32; Hober, '35) and metabolic end products, such as uric acid (Lueken, '32 ; Marshall, '32 a ; Bordley and Richards, '33) and urea (Marshall and Crane, '24; Hober, '30 b, '34; Marshall, '32 b). These substances, besides passing through the glomeruli, are also secreted from the blood into the lumen of the tubules through active work