- Abstract Ephedra sinica Stapf (Ephedraceae) is a widely used Chinese medicinal plant (Chinese name: Ma Huang ). The main active constituents of E. sinica are the unique and taxonomically restricted adrenergic agonists phenylpropylamino alkaloids, also known as ephedrine alkaloids: (1 R ,2 S )-norephedrine (1 S ,2 S )-norpseudoephedrine, (1 R ,2 S )-ephedrine, (1 S ,2 S )-pseudoephedrine, (1 R ,2 S )- N -methylephedrine and (1 S ,2 S )- N -methylpseudoephedrine. GC–MS analysis of freshly picked young E. sinica stems enabled the detection of 1-phenylpropane-1,2-dione and ( S )-cathinone, the first two putative committed biosynthetic precursors to the ephedrine alkaloids. These metabolites are only present in young E. sinica stems and not in mature stems or roots. The related Ephedra foemina and Ephedra foliata also lack ephedrine alkaloids and their metabolic precursors in their aerial parts. A marked diversity in the ephedrine alkaloids content and stereochemical composition in 16 different E. sinica accessions growing under the same environmental conditions was revealed, indicating genetic control of these traits. The accessions can be classified into two groups according to the stereochemistry of the products accumulated: a group that displayed only 1 R stereoisomers, and a group that displayed both 1 S and 1 R stereoisomers. ( S )-cathinone reductase activities were detected in E. sinica stems capable of reducing ( S )-cathinone to (1 R ,2 S )-norephedrine and (1 S ,2 S )-norpseudoephedrine in the presence of NADH. The proportion of the diastereoisomers formed varied according to the accession tested. A (1 R ,2 S )-norephedrine N -methyltransferase capable of converting (1 R ,2 S )-norephedrine to (1 R ,2 S )-ephedrine in the presence of S -adenosylmethionine (SAM) was also detected in E. sinica stems. Our studies further support the notion that 1-phenylpropane-1,2-dione and ( S )-cathinone are biosynthetic precursors of the ephedrine alkaloids in E. sinica stems and that the activity of ( S )-cathinone reductases directs and determines the stereochemical branching of the pathway. Further methylations are likely due to N -methyltransferase activities.