Phenmedipham metabolism in leaf tissue of sugarbeet (tolerant) and rapeseed (sensitive) was compared. Sugarbeet leaf discs metabolized phenmedipham much more rapidly than rapeseed leaf discs, forming two metabolites of relatively low polarity. The less polar of these (metabolite 21) was a precursor to the other (metabolite 11), and its properties indicate derivation from phenmedipham by a single hydroxylation and monoglycosylation. Synthetic N-hydroxyphenmedipham was converted by both species into a compound that cochromatographs with metabolite 21. Purified metabolite 21 was much less inhibitory to light-driven oxygen evolution by isolated thylakoids of both species than was phenmedipham. Hydroxylation/glycosylation without prior carbamate hydrolysis appears to be a major factor in the tolerance of sugarbeet to phenmedipham.