The activity of the angiotensin-converting enzyme (ACE) of the inner surface (the endothelium surface) of rat aorta sections has been studied depending on their distance from the aortic arch, age of rats, and the duration of treatment of rats with the NO synthase inhibitor, N (ω)-nitro-L-arginine (L-NAME). The activity of ACE of aorta sections was determined by measuring the hydrolysis of hippuryl-L-histidyl-L-leucine and was expressed as picomoles of Hip-His-Leu hydrolyzed per minute per square millimeter of the endothelium surface. It was found that the ACE activity considerably varies along the aorta of young rats. This variability decreases with increasing age of rats and by the action of L-NAME. The average ACE activity in the aorta increases with the age of rats and with increasing time of L-NAME treatment. Enalapril normalizes the distribution of the ACE activity along the aorta and decreases the average ACE activity. The changes in the distribution of the ACE activity along the aorta and in the average ACE activity in the aorta with increasing age of the rat and by the action of L-NAME may play a role in the development of atherosclerosis of vessels on aging and the inhibition of formation of nitric oxide.