Objective: This study examined the effects of oral estrogen (ERT) alone versus oral estrogen/medroxyprogesterone acetate (HRT) therapy on cardiovascular function, as controlled by the autonomic nervous system. Study Design: Forty-three postmenopausal women received ERT, HRT, or a placebo for 3 months in a double-blind, randomized, placebo-controlled study. Cardiovascular hemodynamics and heart rate variability were assessed at rest and during stress. Results: After 3 months of therapy, oral HRT significantly increased high-frequency power (P =.0002) and decreased total peripheral resistance (P =.04). The changes were evident at rest and during stress. Conclusion: Our findings suggest that combination therapy produces a more favorable alteration of autonomic cardiovascular function than estrogen alone (ie, combination therapy increases vagal activity). (Am J Obstet Gynecol 2002;186:954-61.)