The oestrogenic component of oral contraceptives affects the activity of liver enzymes and the concentrations of plasma proteins implicated in steroid metabolism and transport. The present study was designed to determine these effects on the kinetics of prednisone and prednisolone. After an oral dose of prednisone, women on oral contraceptive steroids (n = 10) had higher mean (+/- SD) area under the plasma concentration versus time curves of total (428 +/- 67 micrograms/ml/min vs 188 +/- 28 micrograms/ml/min, p less than 0.001) and unbound prednisolone (64 +/- 10 micrograms/ml/min vs 41 +/- 10 micrograms/ml/min, p less than 0.001) than women not taking oral contraceptive steroids (n = 10). The differences were attributable to a lower non-renal clearance of prednisolone and to a higher apparent systemic availability of the drug in contraceptive users than in the controls. The affinity of albumin and transcortin for prednisolone was lower in women on oral contraceptives than in controls (p less than 0.001). Thus, altered kinetics and protein binding may account for the known increase in glucocorticoid efficacy by oestrogens.