Plasma clearance rates and dialysance of labelled cortisol were measured in patients on chronic intermittent dialysis during and after dialysis. Plasma clearance rates during dialysis were 30--63% higher than after dialysis; dialysance was 0.3--0.7 1/h and accounted only for 20--35% of the increase of plasma clearance rates during dialysis. After bolus injection of labelled cortisol, disappearance curves were obtained in two normal subjects and two uraemic subjects on and off dialysis. Biological halftimes of the different segments of the disappearance curves and distribution volumes were calculated. The size of the inner compartment (V1) is greater in uraemia. The disappearance curves flatten after the end of dialysis, indicating an increase of biological halftime after dialysis. Plasma 17-OHCS-glucuronides dropped from 2.13 +/- 0.62 micromol/l (n = 20) at the beginning to 1.05 +/- 0.65 micromol/l at the end of dialysis and increased to 1.51 +/- 0.43 micromol/l (n = 12) within 3 h thereafter. During dialysis cortisol metabolites are removed from the blood. This may decrease inhibition of cortisol metabolism by the end products of cortisol metabolism which might explain the more rapid plasma clearance and the shortened cortisol half-life during haemodialysis.