The purpose of this study was to examine the extent and linearity of dexamethasone binding over a wide concentration range in normal and uremic serum. Tritiated dexamethasone was added to both untreated and charcoal-treated pooled normal serum and to pooled uremic serum to produce concentrations similar to those attained therapeutically (10-1000 ng/mL). Protein binding was determined by equilibrium dialysis at 37 degrees C. Dexamethasone serum binding was linear over the entire range of concentrations for each set of pooled serum. The mean (+/- SD) percent bound (mean +/- SD) for dexamethasone was similar for untreated (75.1 +/- 3.6 percent) and charcoal-treated (77.3 +/- 3.5 percent) normal serum. Dexamethasone binding (69.2 +/- 1.8 percent, p less than 0.05) and serum albumin concentrations (39.9 vs. 55.1 mmol/L) were significantly less in uremic vs. normal serum, respectively. These results suggest that (1) the binding of dexamethasone is linear and occurs primarily to albumin, with little or no binding to corticosteroid-binding globulin; (2) endogenous cortisol does not compete with dexamethasone for protein binding sites; and (3) steroid pharmacokinetics may be altered in uremic patients due to the 24 percent higher free fraction of dexamethasone in this population.