trans-[3-C-13,C-14]Cinnamic acid and the N-acetylcysteamine thioester of [3-C-13,C-14]-trans-cinnamic acid served as precursors of the benzoyl moiety of cocaine when fed to intact Erythroxylum coca plants. The specific incorporation of the thioester into the benzoyl carbonyl group of cocaine was established by means of C-13 NMR spectroscopy. (R)-(+)-[3-C-14]-3-hydroxy-3-phenylpropanoic acid was 11 times more effective than its (S)-(-)-isomer as a precursor of the benzoyl moiety of cocaine. A chemical degradation of the cocaine indicated that all the C-14 was located on its benzoyl moiety. Thus, the stereochemistry of the hydroxy group in 3-hydroxy-3-phenylpropanoic acid, is the same as that in the coenzyme A esters of 3-hydroxy fatty acids which are intermediates in the beta-oxidation of fatty acids.