Dexamethasone in plasma and in tissue is specifically quantitated by high performance liquid chromatography [HPLC] (UV detection at 254 nm) with an octadecyl silane reversed-phase chromatographic column employing peak-height ratio determination (internal standard, cyheptamide). The sample is first washed with heptane under alkaline conditions. The dexamethasone is then extracted from the washed sample with dichloromethane containing the internal standard. Dichloromethane is evaporated to dryness and the concentrated extract is dissolved in tetrahydrofuran and then injected into a HPLC. Dexamethasone and internal standard are eluted with a mixture of acetic acid, methanol, butanol and water (11/19/30/440 by volume). Sensitivity limit is 10 ng, with linear response to at least 1.000 mg/l plasma. Analytical recovery of dexamethasone from plasma is almost complete, and .apprx. 87% dexamethasone is recovered from brain tissue. Intra-assay precision (CV) is 1.07% (n = 11), and interassay precision is 1.38% (n = 5). No interference occurred in plasmas from patients treated with various drugs other than dexamethasone. Dexamethasone was estimated in plasma and in tumor tissue from patients on dexamethasone therapy.