Tritiated thymidine radioautography was employed to study the effect of cortisol and other glucocorticoids on cellular proliferation in the aorta and pulmonary artery of rabbits with cholesterol atherosclerosis. Labelled cell counts showed that glucocorticoids, even after one day and at a relatively low dose, decrease sharply the deoxyribonucleic acid synthesis in the intimal plaques. The hormonal influence on [3H]thymidine uptake seems to be a dose-dependent process. The relative potency of these steroids in inhibiting DNA synthesis in the plaques parallels closely their anti-inflammatory effectiveness. Conversely mineralocorticoids, including aldosterone and deoxycorticosterone, increase the rate of DNA synthesis in the plaques. It is concluded that the antiatherogenic effect of glucocorticoids on cholesterol-fed rabbits may be due, at least partly, to the inhibitory effect of these steroids on the DNA synthesis of the cellular components of the intimal plaques.