Changes in activity of Mg2+- and HCO3- -ATPases were studied in homogenate and mitochondria of liver tissue and myocardium of rats, starving within 1, 2 and 3 days. Mg2+- and HCO3- -ATPase activities were 1.5-2-fold increased in homogenate and mitochondria of the starving rat liver tissue; in myocardium the enzymatic activity was increased 3-5-fold. Effect of fatty acids on the ATPase activity was studied in mitochondria of rat liver tissue and myocardium. Importance of fatty acids in the alteration of the ATPase activity in liver tissue and myocardium of the starving rats is discussed.