The authors studied the ultrastructural appearance and metabolic behaviour of mitochondria in the liver and heart of rats suffering from alloxan diabetes. Liver mitochondria did not show significant morphological and biochemical changes compared to normal, whereas heart mitochondria were decreased in number, their structure was altered and the phosphorylating activity was obviously impaired. Insulin treatment significantly improved the P/O ratio although, as far as the myocardium was concerned, it could not normalize it. The differences in behaviour of liver and heart mitochondria related to the changes of lipid and carbohydrate metabolism and to the disordered ion balance occurring in this experimental endocrine disease, are discussed.