Effects of NaNO(2) and NaNO(3) on the oxidative phosphorylation of mitochondria were studied, and following results were obtained. 1) The effect of NaNO(2) on the oxidative phosphorylation is higher than NaNO(3). 2) Relative activity of State 4 is activated by NaNO(2) (1-10mM) added to reaction mixture. 3) The respiratory control index of mitochondria is decreased by NaNO(2). 4) The effect of NO(2) on oxidative phosphorylation in the borate buffer as reaction mixture is higher than that in the tris-HCL buffer. It is suggested that NO(2) has a probability of the reaction on -NH(2) site of mitochondria.