Kaempferol (3,5,7,4'tetrahvdroxyflavone) inhibited the rate of state 3 substrate oxidation, but not the state 4 rate. This, along with the kaempferol inhibition of substrate-driven calcium-phosphate deposition, provided evidence that kaempferol was acting specifically on the phosphorylation mechanism and not on electron transfer. Kaempferol, however, did not inhibit ATP-driven contraction while oligomycin did. Comparisons of kaempferol with mersalyl indicated that kaempferol did not inhibit phosphorylation by blocking phosphate transport. Both kaempferol and 2, 4-dinitrophenol inhibited calcium-phosphate transport, but kaempferol did not stimulate respiration to the extent that 2, 4-dinitrophenol did under acceptorless conditions. Kaempferol had no effect on NADHdriven contraction in a potassium chloride reaction medium. The site of kaempferol effect is thus seen to be unique from oligomycin and more like aurovertin, likely acting before the formation of the phosphorylated high energy intermediate, but not as an uncoupler in the traditional 2,4-dinitrophenol mode.Flavonoid compounds have been reported to be distributed widely throughout the plant kingdom (11). Despite this wide distribution, little evidence has been presented linking their occurrence with a role in growth or metabolism. Indirect evidence suggesting a role in dormancy and in root and shoot growth has been presented by Harborne (11). It was determined that kaempferol (3,5,7,4'-tetrahydroxyflavone) was a cofactor for in vitro IAA oxidase while quercetin (3,5,7,3',4'-pentahydroxyflavone) inhibited the activity of the enzyme (6). Stenlid (16) conducted an extensive survey of the effects of many flavonoids on ATP production by mitochondria isolated from several plant species. From this work it is evident that minor changes in the hydroxyl pattern affect the degree of inhibition of ATP formation. The work of Stenlid (16) chloroplasts (20). Kaempferol glucosides were a major component of the flavonoids quantitated.Galston (7) has hypothesized from data with quercetin and kaempferol glucosides that flavonoids could play a broadly significant role in contractile and membranous phenomena in plants. Such possibilities would involve transport mechanisms associated with the production and utilization of high energy bonds, most probably as ATP and would thus suggest a possible interaction of mitochondria and flavonoids under certain conditions in vivo. In vitro mitochondrial experiments in which these interactions are studied should be useful in the furtherance of hypotheses such as Galston's (7).When the specific effects of kaempferol on mitochondrial mechanisms for oxidative phosphorylation are being considered, several sites of action are possible as evidenced in studies with other inhibitors. These include the oligomycinsensitive site in some way associated with X P (3), the DNPtsensitive site most likely associated with a proton-motive force or the nonphosphorylated intermediate (2,8,15), or the mersalyl-sensitive site associated with phosphat...