Monuron [3–(p–chlorophenyl)–1,1–dimethylurea] is reported to be a potent inhibitor of the Hill reaction by Cooke and Wessels and van der Veen. It is reported to be 125 times as active as phenylurethan [ethyl–N–phenylcarbamate] in inhibiting the Hill reaction. Diuron [3–(3,4–dichlorophenyl)–1,1–dimethylurea] is 2500 times as active as phenylurethan as a Hill reaction inhibitor. Monuron and diuron have been used in photosynthetic studies. There are no reports in the literature indicating a difference in the mode of action of these compounds.