Simple and reliable methods using LC/MS have been developed for the determination of the b-agonist ractopamine in swine and cattle tissues. Ractopamine was extracted with ethyl acetate from muscle and liver, and the ethyl acetate layer was evaporated to dryness. The residue was purified by partition with acetonitrile/n-hexane. In the case of fat, ractopamine was extracted and purified by partition with acetonitrile/n-hexane. The resulting acetonitrile solutions were evaporated to dryness. The residue was dissolved in methanol, and subjected to LC/MS. The LC separation was performed on a Wakosil-II 3C18HG column (150῍3 mm i.d.) in isocratic mode with 0.05῎ trifluoroacetic acidῌacetonitrile (80 : 20) as a mobile phase at a flow rate of 0.4 mL/min. The MS detection was performed in the selected ion recording (SIR) mode, with detection of the MῌH ῌ ion of ractopamine (m/z 302) produced by electrospray ionization (ESI). The mean recoveries of the drug from swine muscle (0.01 mg/g fortified), fat (0.01 mg/g fortified) and liver (0.04 mg/g fortified) were 99.7῎, 99.5῎ and 100.8῎, and those from cattle samples were 108.3῎, 97.0῎ and 109.4῎, respectively. The relative standard deviations (RSDs) ranged from 0.1῎ to 9.5῎. The limit of quantification (LOQ) of the drug was 1 ng/g.