The effects of OX, a high unsaturated fatty acid fraction extracted from x-ray irradiated rabbit liver, on rat liver mitochondrial swelling was studied and following results were obtained. 1) Strong swelling action is demonstrated at OX concentration above 0.001% in 0.15KCl solution. 2) Swelling action on mitochondria is reduced by sucrose but not changed by sodium succinate. On the other hand, serum albumin and ATP inhibit completely mitochondrial swelling. 3) Mitochondrial swelling caused by OX is reversed by serum albumin and ATP. The intensity of contraction is found in the following order containing ATP, ATP+Mg and ATP +Mg+serum in the media. 4) M itochondrial swelling is induced by 0.005% OX in the substrate of succinate but respiration is not largely prevented. 5) The incorporation of P(32) into Δ10P of swollen mitochondria is almost completely inhibited by OX.