The pharmacokinetics of dexamethasone, given at low dose, were studied in 13 patients with congenital adrenal hyperplasia (CAH) to ascertain whether kinetics differed in this inherited disorder of cortisol metabolism from those seen in healthy individuals. Changes in plasma dexamethasone concentration after intravenous bolus, measured using a simple novel radioimmunoassay, were well described by a two-compartment open model with first-order kinetics. Values for lambda 2: 0.206 h-1, t1/2: 3.53 h, Vc: 24.41 and f: 0.64 were similar to those previously reported for normal subjects. There were considerable interindividual differences in parameter values and Cmaxp.o. (range 22-67 nmol/l). As suppression of the hypothalamo-pituitary-adrenal axis correlates with plasma dexamethasone levels, this variability may partly explain the differing dose and dose schedule requirements necessary to achieve adequate therapeutic control in the clinical management of CAH.