Radioactive khellin, visnagin, and visamminol were recovered from Ammi visnaga plants fed [1-14C]-acetate. A trapping experiment with 5,7-dihydroxy-2-methylchromone indicated that this compound was formed from acetate in the plant. It was then prepared labelled with 14C and 3H and administered to shoots, demonstrating its conversion to visamminol and furanochromones. A biosynthetic pathway is proposed: acetate → 5,7-dihydroxy-2-methylchromone → peucenin →→ visamminol →→ visnagin and khellin.The gas chromatography of some A. visnaga constituents is described. The furanocoumarins xanthotoxin and isopimpinellin were shown to be present in the plant.Administration of labelled 5,7-dihydroxy-2-methylchromone to shoots of Angelica archangelica resulted in the elaboration of labelled furanochromones, suggesting that the furan moiety of furanochromones and furanocoumarins may be formed via similar if not identical enzyme systems.