(1) The concentrations of unconjugated and conjugated phenol, p-cresol, benzyl alcohol, catechol, hydroquinone, homocatechol, and 2-methoxyresorcinol in uremic serum were determined using a mass fragmentographic method. Concentrations of all phenols in uremic serum were higher than in normal serum. Conjugated phenols existed mainly as sulfate esters. (2) The concentration of unconjugated phenol in uremic serum correlated with those of serum creatinine and urea, but not with the hematocrit value. Patients with the highest concentration of unconjugated phenol suffered from cerebral disorders, coma or lethargy, and hemorrhagic tendencies. Unconjugated phenol was detected in normal serum at a concentration of 0.86 +/- 0.63 mumol/l (mean +/- S.D., n = 10). (3) The concentration of unconjugated p-cresol in uremic serum did not correlate with those of serum creatinine, urea and the hematocrit value. (4) The concentrations of unconjugated and conjugated hydroquinone in uremic serum correlated with those of serum creatinine and urea. (5) 2-Methoxyresorcinol was first detected mainly as sulfate ester in uremic serum at a concentration of 19 +/- 9.0 mumol/l (mean +/- S.D,. n = 17). (6) The daily excretion of eight unconjugated phenols into normal urine was determined.