It is shown by C-13 NMR spectroscopy that [carboxyl-C-13]benzoic acid rather than [carbonyl-C-13,H-2]-benzaldehyde supplies the benzylic fragment of the Ephedra alkaloids, norephedrine, ephedrine, norpseudoephedrine, and pseudoephedrine, in growing plants of Ephedra gerardiana sikkimensis. [1,2,3-C-13(3)]-1-Phenylpropane-1,2-dione and (S)-[1,2,3-C-13(3)]-2-amino-1-phenylpropan-1-one([1,2,3-C-13(3)]cathinone) serve as precursors of the Ephedra alkaloids. (R)- and (S)-[1-C-13,1-H-2]-1-hydroxy-1-phenylpropan-2-one are excluded as intermediates. Cathinone is shown to be a constituent of E. gerardiana.