Bergapten and xanthotoxin, labelled in the methyl group with carbon-14 or tritiated at three skeletal carbons, were administered to leaves of Heracleum lanatum and to cell cultures of Ruta graveolens. In all experiments xanthotoxin was the more efficient precursor of isopimpinellin, although bergapten was always incorporated to a measurable extent. Double-labelling experiments showed that both precursors, especially bergapten, underwent considerable demethylation (and presumably remethylation) before conversion to isopimpinellin. 5-Hydroxyxanthotoxin and 8-hydroxybergapten were both O-methylated by cell-free extracts of Ruta cells to isopimpinellin, in reactions mediated by discrete O-methyltransferases. 8-Hydroxy[Me-14C]bergapten was converted with a high degree of incorporation to isopimpinellin by Ruta cells in vivo, and it is suggested that the preference for the pathway via xanthotoxin may be due to more rapid hydroxylation of this substrate.