The monensin residue was extracted from the tissue by homogenization with methanol-water and the extract was filtered and partitioned with dichloromethane. The dichloromethane extract is concentrated and clean up by passing through a silica gel cartridge. The analyte on the cartridge is then eluted with dichloromethane-methanol. The eluate is collected and evaporated to dryness. The residue is dissolved and made to a definite volume with 1 mL methanol and the solution is used for post-column derivatization-HPLC determination. Monensin is separated on mu-Bondapak C18 column (3.9 mm i.d. x 300 mm) with methanol-water-phosphoric acid as a mobile phase and the flow rate was 0.7 mL/min. The eluted monensin was reacted with vaniline under acidic and heated condition in post-column derivatization system then detected at 520 nm and quantitated by external standard method. The derivatization reagent consisted of 20 mL concentrated sulfuric acid, 950 mL methanol and 30 g vaniline. The flow rate was 0.7 mL/min. The reactor was a stainless steel coil (300 cm x 1 mm i.d.) set in a 90 degrees C oven. The response values was linear between 20-200 ng. The recovery was 88.1%-101.3%. The coefficient of variation was 0.1%-0.73%.