A method for measuring the branched chain hydroxy acid beta-hydroxy-beta-methyl butyrate (HMB, a product of leucine catabolism) has been described. A [2H6]HMB internal standard was added to plasma and standards, and samples were extracted with diethyl ether, backextracted into neutral phosphate, dried, and derivatized for gas chromatography and mass spectrometry. The natural HMB was monitored at 175 amu and the deuterated HMB was monitored at 181 amu. Standard curves were linear to at least 25 microns and were quantitatively recovered from plasma. Basal concentrations of plasma HMB were from 1 to 2 microM in sheep and increased three- to fourfold when leucine's alpha-ketoacid (alpha-ketoisocaproate, KIC) was fed to lambs. This method can also be adapted to quantitate KIC and other branched chain ketoacids in plasma during the same run.