The effects of fatty acids on swelling and contraction of mitochondria taken from rat liver and Ehrlich ascites tumor cells are studied and the following results were obtained. 1. The swelling of rat liver mitochondria is induced most effectively by myristic acid (14carbon chain compound) among the saturated fatty acids and by cis form unsaturated fatty acids more than trans form. These effects are proportional to the degree of stimulation on latent ATPase activity by fatty acids. 2. The degree of swelling of rat liver mitochondria is proportional to a concentration of oleic acid and is inhibited by anaerobiosis, cyanide, azide or antimycin A. These swollen mitochondria can be contracted reversibly by ATP, bovine serum albumin and magnesium ion. 3. The swelling action of oleic acid is stronger in the following order on mitochondria taken from mouse liver, liver of Ehrlich ascites tumor bearing mouse, Ehrlich ascites tumor cells and solid tumor. The degree of swelling parallel to respiratory activities in those mitochondria respectively. 4. The increase of respiration, decrease of ATP synthesis and inhibition of oxidative phosphorylation are observed in swollen mitochondria induced by oleic acid. 5. The amount of phosphoprotein and phospholipid in mitochondria decreases by oleic acid and the turnover of phosphate in bothe compounds also decreases. 6. The amount of pottasium in mitochondria increases slightly soon after adding of oleic acid but rather decreases later on.