The basic factor of cardiovascular diseases is atherosclerosis, which is due largely to an increase in the activity of the angiotensin-converting enzyme (ACE) in vessels. Flavonoids diminish the risk of cardiovascular diseases and the flavonoid taxifolin normalizes the activity of ACE. We examined the efficiency of seven flavonoids in preventing an increase in ACE activity in aorta of rats exposed to ionizing radiation. It was shown that the activity of flavones and flavonols decreases with an increase in the number of OH groups in the A and B rings, respectively. The reduction in the activity of flavonoids within the classes correlates with a decrease in their lipophilicity. Flavanonols (taxifolin) are more active than flavonols, and flavonols are more active than flavones.