Companion studies were designed to determine the effects of oral contraceptives and conjugated estrogens on the pharmacokinetics of prednisolone. Twenty-four normal women entered the studies, including six young women taking oral contraceptives and six age-matched control women, and six postmenopausal women receiving conjugated estrogens and six age-matched postmenopausal women. All received 0.53 mg/kg prednisolone phosphate, iv. Significant decreases (P less than 0.05) in the clearance and volume of distribution and significant increases in the half-life were found for both total and unbound prednisolone in the women taking oral contraceptives compared to values in the young control women. A significant decrease in the unbound clearance and increases in the total and unbound half-lives of prednisolone were found in the women receiving conjugated estrogens compared to values in the postmenopausal control women. Total clearance and volume of distribution were unchanged by conjugated estrogen therapy. Administration of prednisolone to women receiving estrogen-containing oral contraceptives or conjugated estrogens results in exposure of these women to increased concentrations of unbound prednisolone for increased periods of time. Increases in the pharmacological and toxic effects of prednisolone might be expected in these women.