Values of PAH and Radio-Hippuran clearances, obtained simultaneously in 96 individual renal clearances for 16 patients, were plotted against each other and a regression line calculated which shows a linear relationship between the two substances. Knowing the value of the Radio-Hippuran clearance, by use of the regression line, the effective renal plasma flow in terms of PAH clearance can be estimated. This method of determination gives an accurate and less laborious measure of renal plasma flow than chemical determinations of PAH. Submitted on July 21, 1961