The rate of flow of electrons from water to an artificial electron acceptor, dichlorophenolindophenol (DCPIP), through photosystem II in the thylakoid membranes of isolated chloroplasts is greatly enhanced in the presence of 10 mM ammonium chloride. Rate enhancement depends on irradiance levels. Uncoupling reagents like ammonium chloride prevent the formation of a proton gradient across the thylakoid membrane and consequently remove a constraint on the rate of electron transport. The mode of action of ammonium chloride is explained. Evidence obtained using an oxygen electrode that DCPIP itself also partially uncouples the system is presented as background information for instructors. Suggestions on how this reaction may be used in laboratory classes for students from high school to the senior undergraduate level are included.