Background: Estrogen blunts the neointimal proliferative response to balloon injury of the carotid artery in intact female rats and gonadectomized rats of both sexes. This study tested whether, in gonadectomized rats of both sexes, (1) progestin (medroxyprogesterone acetate, MPA) alters neointima formation in injured carotid arteries, (2) addition of MPA alters the antiproliferative effects of estrogen, and (3) an interaction between MPA and estrogen can be accounted for by MPA-induced alterations in serum 17β- estradiol levels. Methods and Results: Male and female Sprague-Dawley rats were subjected to gonadectomy, then were randomly divided into four subgroups and treated with either (1) 17β-estradiol, (2) MPA, (3) 17β-estradiol + MPA, or (4) vehicle, and balloon injury of the right common carotid artery was carried out. Two weeks later, rats were killed by overdose of pentobarbital, and the carotid arteries were subjected to morphometric analysis for evaluation of myointimal thickening. Estradiol inhibited myointimal proliferation after vascular injury in gonadectomized rats of both sexes (P<.05). MPA alone did not alter neointima formation, but addition of MPA to estradiol completely blocked the antiproliferative effects of estrogen without altering serum 17β-estradiol levels. Conclusions: These data indicate that exogenous progestin given alone does not alter the vascular injury response in the rat carotid injury model but that addition of a progestin blocks the antiproliferative effects of estrogen in this model. These effects are seen in gonadectomized rats of both sexes. These findings have direct implications for postmenopausal hormone replacement therapy in humans.