Diabetes was introduced in rats by an intravenous injection of streptozotocin (65 mg/kg). Animals were sacrificed 8 weeks later and mitochondria were isolated from the ventricular tissue by differential centrifugation. The state 3 respiration, oxidative phosphorylation rate and Mg2+-dependent ATPase activities were depressed in mitochondria from diabetic hearts. These changes were partially reversible upon 2 weeks of insulin and fully reversible after 4 weeks of insulin therapy. Mitochondrial calcium uptake but not calcium binding, was decreased in diabetes and this change was fully reversible by 2 weeks of insulin administration. The observed alterations in mitochondrial function could not be explained on the basis of any changes in mitochondrial lipid and protein composition or subcellular contamination. These results indicate the presence of a generalized depression in mitochondrial function in chronic diabetes and such a defect is suggested to contribute in the development of cardiomyopathy at late stages of diabetes.