Bioassay-guided fractionation of the ethyl acetate extract of Ruta graveolens (common rue) leaves led to the isolation of the furanocoumarins 5-methoxypsoralen (5-MOP), 8-methoxypsoralen (8-MOP), and the quinolone alkaloid graveoline as phytotoxic constituents. Graveoline and 8-MOP substantially inhibited growth of Lactuca sativa (lettuce) seedlings and reduced chlorophyll content at 100 microM; this effect was not due to a direct effect on chlorophyll synthesis. Radical growth of L. sativa was inhibited by 10 microM 8-MOP. Graveoline inhibited growth of Lemna paucicostata (duckweed) at 100 microM. This is the first report of the phytotoxic activity of graveoline. Growth of Agrostis stolonifera (bentgrass) was inhibited by 5-MOP at 30 microM. All three compounds substantially reduced cell division in Allium cepa (onion) at or below 100 microM. None of the compounds caused significant cellular leakage of Cucumis sativus (cucumber) cotyledon disks at 100 microM. All three compounds inhibit plant growth, at least partially through inhibition of cell division.