This is the first paper on the mode of action of a new class of broadly active foliar herbicides, 2-methylcinnoliniums. One of the most active compounds of this class is 2-methylcinnolinium-4-(O-methyl phosphonate) (MCP). The symptoms of phytotoxicity include desiccation, chlorosis, and necrosis, and are very similar to those caused by methylviologen. Both MCP and methylviologen have reversible oxidation-reduction waves as measured by cyclic voltammetry, and both molecules have reduction potentials that are sufficiently positive to compete for electrons with the endogenous electron acceptors of photosystem I (PS I). MCP is a highly active inhibitor of whole-chain electron transport, similar to methylviologen, with an I50 of about 25 μ. This compound is not an inhibitor of photosystem II (PS II). However, when PS II is inhibited by diuron, partial-chain electron transport from the artificial electron donor DPIPH2 to NADP is still strongly inhibited (I50 = 40 μ) by this molecule. We conclude that MCP acts in the same fashion as methylviologen (Bowyer, J. R.; Camilleri, P. In Herbicides·, Hutson, D. H., Roberts, T. R., Eds.; Wiley: New York, 1987; pp 105-145), by competing with endogenous acceptors for electrons that would normally flow through the reducing side of PSI to NADP.