1. The volatile phenols in sera of uraemic patients were analysed by gas-liquid chromatography. The concentrations of p-cresol, 1·17 ± 0·58 (mean ± SD), and phenol, 0·44 ± 0·39 mg/100 ml, were higher than the corresponding values (0·23 ± 0·17 and 0·02 ± 0·03 mg/100ml, respectively) in control subjects. Both compounds were recovered mainly as acid labile conjugates, probably sulphate esters. No unconjugated phenol, o-, m-, or p-cresol or conjugated derivatives of o- or m-cresol were detected.
2. When the standard hospital diet was replaced by an isocaloric low-protein diet, the concentration of p-cresol and urea in serum decreased in the two uraemic patients studied. The serum concentration of phenol was uninfluenced by this change of diet.
3. One female patient was studied during treatment with peritoneal dialysis, which on two out of three occasions appeared to result in decreased concentrations of phenol and p-cresol in serum.