H^+-ATPases located on plasma and vacuolar membranes play major roles in various cellular physiological processes. In order to investigate the physiological roles of H^+-ATPases, microsomes were prepared from tomato roots and the effects of various anions were measured on the activities of H^+-ATPases. H^+-ATPase was inhibited by various anions. Citrate and phosphate were chosen to investigate detailed inhibitory mechanisms on H^+-ATPases since they showed different levels of inhibition. Inhibitory effect of citrate was observed at the concentrations above 3 mM. When 20 mM citrate was added, the ATPase activity was decreased by 50-60%. However, the inhibitory effect of citrate was decreased by increasing the concentration of Mg^(2+). The citrate-induced inhibited activity was recovered by the addition of Mg^(2+). Addition of 7 mM Mg^(2+) completely removed the inhibitory effect of citrate and the activity recovered to the level of the control experiment. These results imply that citrate chelates Mg^(2+) and thus inhibits H^+-ATPases. Meanwhile, the inhibitory effect of phosphate was observed at the concentration above 3 mM and the activity was decreased by 50% in the presence of 30 mM phosphate. Further addition of Mg^(2+) showed no recovery on the activity. These results imply that the inhibitory effect of phosphate is not dependent upon the concentration of Mg^(2+).