MANY ORGANIC acids accumulate in renal failure.1The role of phenolic acids in the pathogenesis of uremia has been reviewed by Schreiner and Maher2and others.3-10Blood phenols are primarily derived from the action of intestinal bacteria on protein derivatives containing aromatic amino acids. They are absorbed from the gut, conjugated in the liver, and excreted principally by the kidneys. Normally total plasma phenols including various amino acids, phenols, and aromatic hydroxy acids do not exceed 0.1 mg/100 ml. Marked elevations of free and conjugated phenols have been detected in uremia and reported to correlate with uremic symptoms, especially those of central nervous system (CNS) depression.3-6 The present study was undertaken to evaluate in vivo the potential contribution of the phenolic compound, 3,4-dihydroxybenzoic acid (3,4-DHBA) (protocatechuic acid) to the pathogenesis of uremia. This compound was selected because it is excreted in considerable amounts (46