A rapid and sensitive microbore high-performance liquid chromatography (HPLC) coupled to microdialysis technique for simultaneous determination of unbound hesperetin in rat blood and brain has been developed. The HPLC assay was carried out using a microbore reversed-phase C-18 column (100b1 mm I.D., 5mm particle size). The mobile phase used was acetonitrile-methanol-20 mM monosodium phosphate (pH 5.3) (40:10:50, v/v/v) for hesperetin at a flow-rate of 0.05 mL/min. The assay was performed by monitoring the wavelength of maximum UV absorbance at 288 nm. Microdialysis probes were inserted into the jugular vein/right atrium and brain striatum of male Sprague-Dawley rats. Hesperetin (50 mg/kg, i.v., n=6) was administered via the femoral vein, and microdialysates were collected from both sites and assayed by the validated microbore scale HPLC method. Pharmacokinetic parameters were calculated from the corrected data for dialysate concentrations of hesperetin versus time.