The activities of hexokinase, glucose-6-phosphate dehydrogenase, hydroxyacyl-CoA-dehydrogenase, adenylate kinase and glutathione reductase were determined in the aorta of rats made diabetics with streptozotocin for over two weeks and in noninjected controls. Adenosinetriphosphate (ATP) and total adenine nucleotide content were also measured. Glutathione reductase activity was not significantly changed in the diabetic aorta whereas the activities of glucose-6-phosphate dehydrogenase, hydroxyacyl-CoA-dehydrogenase and adenylate kinase were all increased. Hexokinase activity was significantly decreased in diabetic rat aorta. When measured after incubation in vitro for 2 h with 5.6 mmol/l glucose, the ATP-concentration was reduced in the diabetic aorta while the total concentration of adenine nucleotides was unchanged. Insulin treatment started three days after induction of diabetes with streptozotocin and continued for twelve days restored the growth rate of the rats but their glucose metabolism was not completely normalized. After insulin treatment no significant differences between diabetic and normal rats were found in the aortic activities of glucose-6-phosphate dehydrogenase, hydroxyacyl-CoA-dehydrogenase, adenylate kinase or in the ATP content.