The pharmacokinetics of prednisone and prednisolone was examined in 12 healthy male subjects to assess the bioavailability and the parameters of reversible metabolism between the two steroids. After an oral prednisone dose of 0.8 mg kg-1 and an intravenous prednisolone dose of 0.66 mg kg-1, the bioavailability was found to be about 62%. The fraction of the dose recovered in the urine as the hydroxylated metabolites of prednisone and prednisolone was lower after the oral prednisone dose, suggesting that poor absorption of prednisone was the main cause of the low bioavailability. There was a high degree of interconversion between prednisone and prednisolone with 76% of the dose being recycled. The formation clearance of prednisolone from prednisone is much greater than the formation clearance of prednisone from prednisolone or the irreversible elimination clearances of the two steroids. The possible dose dependences of bioavailability and interconversion may be important factors in prednisolone therapy.