Our approach serves several purposes: To demonstrate the response of aortic cell nuclei to a changing environment (eg hypercholesterolemia, experimental diabetes, drug treatment), to apply the acridine orange (AO) ultracytochemical method, and to determine the euchromatin/heterochromatin (EU/HET) ratio by morphometric procedures as suitable criteria, and to emphasize the differential nuclear activity of adventitial cells. Hypercholesterolemia in rabbits results in an increase of AO positive nuclei in the adventitia. Additional treatment with tinofedrine, a new cerebral vasodilator drug, reveals an even higher number of AO positive nuclei in adventitial and also in endothelial and plaque cells. In streptozotocin induced diabetes in rats the increased number of AO positive cells coincides with an elevated EU/HET ratio.