The purpose of this study was to determine whether supplemental estrogen influences cardiovascular hemodynamics at peak exercise in endurance trained and sedentary postmenopausal women. Subjects were 22 women between 3 and 10 yr past menopause who had engaged in endurance exercise at least three times per week for one or more years. Twelve of the women had taken estrogen replacement for at least 1 yr (ER) while the other 10 had never taken supplemental estrogen (NOER). Peak cardiac output (˙Qpeak) and, subsequently, peak cardiac index (˙QIpeak) were calculated by regressing submaximal cardiac output values on corresponding oxygen consumptions and extrapolating to peak exercise. Peak oxygen consumption in the two groups were almost identical; however, the ER group demonstrated a higher˙QIpeak in conjunction with a lower arteriovenous oxygen difference and a lower peripheral resistance. It was concluded that estrogen supplementation may be associated with higher peak cardiac outputs in exercise trained postmenopausal women via alterations in the peripheral vascular and oxygen kinetic responses to maximal exercise.