Summary:Chlorophyll production by wheat (Triticum aestivumL. ‘Mericopa’) and corn (Zea maysL. Merit) was severely curtailed by the experimental herbicide l‐(m‐trifluoromethylphenyl)‐3‐chloro‐4‐chloromethyl‐2‐pyrrolidone (R‐40244). A 96% loss in chlorophyll content was observed in wheat seedlings treated with 320 μmol/1 R‐40244. In corn 320 μmol/1 R‐40244 caused a 40% drop in chlorophyll content. Carotenoid production in both wheat and corn was inhibited by R‐40244, 320 μmol/1 of the herbicide causing a 97% reduction in wheat and a 44% reduction in corn. The inhibition of carotenoid production which, in turn, allows chlorophyll to be photooxidized, may represent the mode of action of R‐40244. Hill reaction activ