The pharmacokinetics of prednisolone elimination have been studied in both arthritic patients and normal volunteers using tritiated prednisolone alone, and in conjunction with unlabelled prednisolone in doses of 0.15 mg-kg-1 and 0.3 mg-kg-1 body weight. With increasing dose there is prolongation of the plasma half-life and increase in the volume of distribution and plasma clearance of prednisolone. It is proposed that these changes in pharmacokinetic parameters may be associated with non-linear binding of the steroid to plasma proteins.