p-Aminohippuric acid (PAH) is a derivative of hippuric acid and useful as a diagnostic agent for the measurement of renal plasma flow. About 20-30% is completely filtered by the glomerulus and not readsorbed by the tubules. The remainder which bypasses the glomerulus and enters the tubules is completely secreted. At low doses, PAH is almost completely removed with one-pass through the kidneys. Hence, the venous concentration of PAH is close to zero and PAH has been used to determine the effective renal plasma flow (eRPF) from the plasma. The venous concentration is usually <10% that of the plasma concentration so eRPF slightly underestimates the actual RPF. This error is generally accepted because of the ease with which PAH infusion allows