Mitochondria from heart of rats fed a diet rich in rapeseed oil for two months from weaning, oxidized oleic acid a little better than those from animals fed a control diet containing peanut oil; but their activity decreased much more in presence of potassium cyanide which is an oxidation inhibitor. When observed by electronic microscopy, mitochondria from heart of rats fed rapeseed oil were more mumerous and more mitochondria of great size were encountered as compared with control rats. Biochemical data seemed to show a decrease of activity or quantity of certain enzymatic systems in mitochondria from heart of animals fed rapseed oil. Cyanide could more easily inhibit these enzymatic systems because of their deficiency. It can be thought that it is the mitochondria of great size which are implicated in this phenomenon since their growth is not accompanied by an increase of their content in certain enzymes which are just sensitive to cyanide. A mixture richer in mitochondria of big size, thus poorer in certain enzymes for an equal protein content in drastic conditions, such as partial anoxia since cyanide blocks the respiratory chain.