The effects of bicarbonate, sulfite and thiocyanate anions on the ATPase activity of mouse liver mitochondria were studied. The anions stimulated and inhibited the ATPase activity non-competitively with respect to ATP and Mg2+. The stimulatory and inhibitory effects of the anions were persistent, when the ratios of Mg2+ and ATP wee changed considerably. Thyocyanate inhibited the ATPase activity competitively to sulfite. The hydrolysis of ATP was sensitive to sulfite, bicarbonate and thiocyanate in the presence of calcium, cobalt, manganese and zinc. The correlation between the data obtained and the previously proposed mechanism of the anions effect of the ATPase activity is discussed.