The biosynthesis of cannabinoids was studied in cut sprouts of Cannabis sativa by incorporation experiments using mixtures of unlabeled glucose and [1-(13)C]glucose or [U-(13)C(6)]glucose. (13)C-labeling patterns of cannabichromenic acid and tetrahydrocannabinolic acid were analyzed by quantitative NMR spectroscopy. (13)C enrichments and coupling patterns show that the C(10)-terpenoid moiety is biosynthesized entirely or predominantly (> 98%) via the recently discovered deoxyxylulose phosphate pathway. The phenolic moiety is generated by a polyketide-type reaction sequence. The data support geranyl diphosphate and the polyketide, olivetolic acid, as specific intermediates in the biosynthesis of cannabinoids.