The effect of water extracts of cinnamon and clove on rat mitochondrial F(0)F1ATPase was investigated. Both spices stimulated ATP hydrolysis. Gas chromatography analysis of the water extracts, confirmed the presence of eugenol and cinnamaldehyde as major components in clove and cinnamon, respectively. Both components (1) stimulated ATPase significantly at concentrations equal or greater then 0.3 mM; (2) reduced mitochondrial membrane potential: a 50% decrease in Deltapsi was obtained at 7.56 and 11.6 micromoles/mg protein for eugenol and cinnmaldehyde, respectively; (3) inhibited NADH oxidase or complex I of the respiratory chain with a 50% inhibition at 15 and 20 micromoles/mg protein for eugenol and cinnamaldehyde respectively; (4) had no effect on succinate dehydrogenase activity. The study proposes the mitochondria as a target for the action of the spices resulting in derangement of mitochondrial functions, particularly at proton transferring sites.