The plasma concentration and renal elimination of phenols was studied in 32 individuals with various renal insufficiency (CRI) and in 30 healthy subjects. In patients with chronic renal insufficiency the increase in P phenols values correlated directly with Per and P urea. Daily urinary excretion of phenols in patients with CRI is only mildly decreased compared with that in healthy controls. Renal clearance (C phenols), 8.52 (+/- 2.69) ml/min on an average, decreases significantly in patients with CRI. While fraction excretion of phenols (FE phenols) was 9.53 (+/- 4.14) % on an average in healthy persons, patients with CRI displayed a significant increase. A significant linear correlation in the values of FE phenols and FEH2O and those of FE phenols and FE urea was documented. Our findings support the assumption that phenols are excreted by the kidney by a mechanism similar to that of urea excretion. Filtration of phenols in the glomeruli is followed, in healthy volunteers, by their significant reabsorption which is a flow-dependent process. In the residual nephrons of patients with CRI, the tubular reabsorption of phenols is decreased, a mechanism largely compensating the effect of decreased filtration of phenols on their total urinary excretion.