Four substituted pyridazinone compounds inhibited the Hill reaction and photosynthesis in barley (Hordeum vulgare L., var. Dayton C.I. 9517). These inhibitions appeared to account for the phytotoxicity of 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon). The pyridazinone chemicals were weaker inhibitors than 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine). Two substitutions onto the molecular structure of pyrazon result in a new experimental herbicide, 4-chloro-5-(dimethylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone (hereinafter referred to as 6706), which retains the action mechanism of pyrazon but also has two additional biological properties. It is resistant to metabolic detoxication in plants, and it possesses a second mode of action involving interference with chloroplast development. The second action is like that expressed by 3-amino-s-triazole (amitrole) and by 3,4-dichlorobenzyl methylcarbamate (dichlormate). However, the new chemical is 100 to 1000 times more effective. The trifluoromethyl substitution on the phenyl ring and the dimethyl substitution on the amine are both required to give either of the two additional physiological properties. Analogs with only one of the two substitutions behave like pyrazon rather than like 6706.