A selective capillary zone electrophoresis (CZE) microassay was developed for the simultaneous determination of dexamethasone phosphate and its major metabolite, dexamethasone, in tears. The calibration was carried out in the biological matrix with indoprofen as an internal standard which allowed the separation of dexamethasone phosphate and dexamethasone from the tear constituents. The limits of detection and quantification of the assay were 0.5 and 2.0 microg ml(-1), respectively. This quantification method is essential for the in vivo determination of dexamethasone concentration-time profiles in tears after application of the antiinflammatory drug.