Present evidence indicates that proteinuria in patients with the nephrotic syndrome is the result of increased permeability of the glomerular capillary walls to proteins, particularly to albumin (1, 2). A rough indication of the permeability to albumin, relative to permeability to water, is provided by the ratio of the concentration of albumin in glomerular fluid to that in plasma. Assuming that no albumin is excreted by the tubules, the rate of albumin excretion (in mg. per min.) divided by the rate of glomerular filtration of water (GFR, in ml. per min.) represents the lowest possible concentration of albumin in glomerular fluid (in mg. per ml.) ; if some albumin is reabsorbed by the tubules, as seems likely, the actual concentration of albumin in glomerular fluid would be greater than this calculated value. If it is accepted that the clearance of inulin (CIN) is equivalent to GFR in children with the nephrotic syndrome (3), it follows that the renal clearance of albumin (CALB) divided by CIN, i.e., CALB/CIN, represents the minimum ratio of the albumin concentration in glomerular fluid to that in plasma. Chinard, Lauson, and Eder (4) showed that in patients with the nephrotic syndrome the renal clearance of the blue dye, T-1824 (CT_1824), was somewhat less than but approximately proportional to CALB. It would appear, therefore, that where large changes in permeability are expected CT_1824should be a satisfactory substitute for CALB. The