The primary purpose of the study was to examine sex differences and menstrual cycle time-points on chemoreflex function during supine and 70o upright (HUT) positions during: 1) normoxia, 2) hypercapnia (5% CO2), or 3) hyperoxia (100% O2). Women were tested during the early-follicular phase (EF; days 2-5) and the mid-luteal phase (ML; days 18-24). Compared to baseline, men and women had lower cardiac output index (Qi), mean arterial pressure (MAP), cerebrovascular resistance index, and respiratory rate during HUT. In response to hypercapnia during HUT (compared to supine), men had an augmented increase in MAP, while all groups had an augmented increase in ventilation suggesting sexually dimorphic interactions between the baroreflex and central chemoreflex. In response to hyperoxia during HUT, men and women displayed an attenuated increase of total peripheral resistance index and an attenuated decrease of Qi suggesting upright posture activated peripheral chemoreceptors
