Sodium fluoride (6-8 mg/ml) inhibits hydrolysis of methylprednisolone acetate to methylprednisolone. An HPLC (high pressure liquid chromatography) method for simultaneous determination of hydrocortisone, methylprednisolone and methylprednisolone acetate in plasma was presented. Analysis of plasma samples (containing NaF) for methylprednisolone acetate shows no significant change in concentration over extended periods of storage at -20.degree. C. In vitro hydrolysis of methylprednisolone acetate at 37.degree. C in human whole blood was rapid (average T1/2 (half-time) = 29 min). In 1 cat, the bioavailabilities of methylprednisolone acetate rectally was 13% and of methylprednisolone (alcohol) rectally was 26%, relative to i.v. administration of methylprednisolone. In the same cat, the bioavailabilities of methylprednisolone acetate orally was 93% and of methylprednisolone was 82%, relative to i.v administration of methylprednisolone. All samples collected after oral adminsitration of methylprednisolone acetate to a human subject had only methylprednisolone (alcohol) indicating hydrolysis of the drug during absorption through the gastrointestinal membrane and/or in the liver. If the ester had the same half-life in blood in vivo as measured in vitro, it would have been measurable in plasma.