The effect of sodium nitrite on the mitochondrial latent ATPase activity was studied and the following results were obtained. 1. Sodium nitrite stimulated the latent ATPase activity, and its stimulating activity was dependent on its concentration added to the reaction mixture. 2. The activity of the ATPase stimulated by sodium nitrite was lower in the tris buffer having the amino group than that in the borate buffer. 3. The ability of sodium nitrite on release respiratory control was lower in the high concentration of mitochondrial protein than that in the low concentration of protein.