On the basis of direct quantification of hemodialysis (HD), the kinetics of phenols (Ph) were followed in 13 patients on regular HD treatment. The average plasma levels of Ph before and after HD were 627 +/- 109 mumol/L and 416 +/- 81 mumol/L, respectively. The total amount of Ph removed during 5-h HD was 7,481 +/- 1,894 mumol. For calculation of the generation rate (G), a new formula has been derived not requiring knowledge of the corresponding volume of distribution. The G of Ph was 2.9 +/- 0.7 mumol/min on average. The mean dialysis clearance (K) of Ph was 48.2 +/- 10.2 ml/min.