The mutual effects of Cd(2+), ruthenium red and Cd(2+) on the rat liver mitochondrial latent ATPase activity were studied and the following results were obtained. (1) The mitochondrial latent ATPase activity was stimulated by Cd(2+), and higher concentration of Cd(2+) added to the reaction mixture showed higher ATPase activity. (2) The ATPase activity induced by Cd(2+) was inhibited by ruthenium red, having the high affinity for the Ca(2+) -binding site of mitochondria. (3) The ATPase activity induced by Cd(2+) was also inhibited by Ca(2+). (4) In conclusion, the site of mitochondria, on which Cd(2+) activates latent ATPase, will be related with the Ca(2+) -binding site of mitochondria.