A decrease in the ATPase (heat stable) and 5-nucleotidase of atherosclerotic regions in human aortae has been demonstrated. An increase in the breakdown of TPN, as measured by phosphate release, of the atherosclerotic aorta as a whole has been shown. No change was observed in the breakdown of DPN, as measured by phosphate release, between the normal or atherosclerotic aorta. No reactivity for glucose-1 or glucose-6-phosphate or β-glycerophosphate was found in the intima or media. The possible implications in the development of atheroma are discussed.