PAH IS FILTERED BY THE GLOMERULI AND IS ACTIVELY SECRETED BY THE PROXIMAL TUBULES. AT LOW PLASMA CONCENTRATIONS (1.0 TO 2.0 MG/100 ML), AN AVERAGE OF 90 PERCENT OF PAH IS CLEARED BY THE KIDNEYS FROM THE RENAL BLOOD STREAM IN A SINGLE CIRCULATION. IT IS IDEALLY SUITED FOR MEASUREMENT OF ERPF SINCE IT HAS A HIGH CLEARANCE, IS ESSENTIALLY NONTOXIC AT THE PLASMA CONCENTRATIONS REACHED WITH RECOMMENDED DOSES, AND ITS ANALYTICAL DETERMINATION IS RELATIVELY SIMPLE AND ACCURATE.