p-Aminohippurate (PAH) clearance studies were carried out on 15 patients suffering from renal disorders of different origin. In 6 patients the two kidneys were tested separately. It could be shown that in the human organism PAH undergoes partial metabolization resulting in the production of p-aminobenzoate (PAB), N-acetyl-PAH and N-acetyl-PAB. Distribution pattern of PAH and its metabolites is different in urine and blood samples: there is more N-acetyl-PAH in the urine than in the blood. Benzoates make up to 75% of all aromatic amines present in the renal vein. Since routine laboratory analysis does not detect the metabolites, the theoretical basis of applicability of PAH as a clearance substance is invalidated. Re-absorbed benzoates may be one of the causes of incomplete extraction ratio of PAH. Renal artery stenosis reduces the capability of the human kidney to N-acetylate aromatic amines.