OBJECTIVE Estrogens have been shown to protect against cardiovascular disease in postmenopausal women. The mechanisms are unknown. In this study we investigated the effect of estrogen treatment on arterial neointimal proliferation elicited by balloon injury of blood vessels of the rabbit. METHODS The aorta and the common and external iliac arteries of the rabbit underwent balloon injury. 17-beta-Estradiol cypionate (100 micrograms/kg/day intramuscularly) was administered beginning 1 day before injury and until sacrifice at 22 to 24 days after injury. Angiopeptin (20 micrograms/kg/day) was administered for the same length of time as estrogen to a group of rabbits to use this peptide as a positive control for morphometric analysis. Angiopeptin was also combined with estradiol to determine whether it was possible to further enhance the effect of estrogen. Morphometric studies were performed to determine measurement of intimal thickening. Inhibition of cell proliferation by estrogen was evaluated by incorporation of tritiated thymidine in vitro into the balloon injured rabbit aorta 72 hours after balloon injury. RESULTS Treatment of male rabbits with estradiol significantly (p < 0.01) decreased the neointimal thickening of these vessels by 50% to 70%. The somatostatin analog, angiopeptin, was similarly effective in the same circumstances. Estradiol failed to further inhibit neointimal thickening when combined with angiopeptin. Treatment with estradiol for 3 days inhibited both thymidine incorporation (p < 0.01) and DNA content in injured vessels. CONCLUSIONS Estradiol treatment of rabbits undergoing balloon injury of the aorta and iliac arteries, significantly inhibits the myointimal thickening. This effect of estrogen is mediated by inhibition of vascular smooth muscle cell proliferation.