A sensitive, selective, and reproducible high-performance liquid chromatographic assay for the simultaneous measurement of cortisol and methylprednisolone using dexamethasone as the internal standard is presented. Samples are extracted with methylene chloride, washed with sodium hydroxide and then water, and chromatographed on a microparticle silica gel column with ultraviolet detection at 254 nm. Sensitivity is greater than 10 ng/ml and the intra-day coefficient of variation is less than 5% for both steroids. The use of porcine liver esterase allows the quantitation of the hemisuccinate ester of methylprednisolone. This assay has been applied in pharmacokinetic studies including investigations of troleandomycin--methylprednisolone interactions. A typical plasma concentration--time profile for methylprednisolone and its ester prodrug is presented for one subject before and after receiving troleandomycin therapy. Although methylprednisolone elimination is reduced in the presence of troleandomycin therapy, there is no effect on the pharmacokinetics of methylprednisolone sodium succinate.