Solid-phase micro extraction (SPME) and on-fiber derivatization followed by Gas Chromatography coupled with Flame Ionization Detection (GCFID) or Selected Ion Monitoring Mass Spectrometry (GCSIMMS) allows for simple yet sensitive quantification for the hexamethyldisilazane derivative of the -agonist clenbuterol. Using an 85-m polyacrylate fiber, the analysis method is optimized with respect to extraction time, derivatization time and temperature, and solution pH. In addition, the use of a rapid temperature ramping injection port allows for optimization of fiber desorption conditions. Under optimal conditions, the limits of detection for the hexamethyldisilazane derivative of clenbuterol are 1.1ppb by FID and 0.20ppb by SIMMS.