The aim of our study was to determine if cerebral blood flow regulation in women is affected by the menstrual cycle, which could lend insight into whether hormone levels in women may affect their susceptibility to cerebrovascular injury. Four young (mean age 22 ±1.4 years) healthy women were tested at three time points; during menstruation (M), late follicular (F), and the luteal phase (L) of the menstrual cycle. Each visit consisted of a cerebrovascular reactivity test and a sit-to-stand test. Beat-by-beat blood pressure, ECG, PETCO2, and transcranial doppler sonography of the middle and anterior cerebral artery were taken for each subject. During the F phase compared to the other phases, average heart rate was significantly lower (M: 76±1; F: 69±1; L: 73±2 bpm, P=0.026), mean cerebral flow velocity while standing was significantly lower (M: 100±2; F: 94±1; L: 99±1 %, P=0.003), and cerebrovascular resistance increased while standing (M: 0.03±0.01; F: 0.06±0.02; L: 0.01±0.06 mmHg/%, P=0.049). In contrast, blood pressure changes during standing were unaffected by cycle phase. Examining cerebral autoregulation, autoregulatory index values were higher during the menstrual phase with a tendency for decreasing scores during F and L in the MCA (M: 4.4±0.6; F: 3.4±0.4; L: 2.0±0.6, P=0.032) but not the ACA (M: 4.2±0.6; F: 3.4±0.4; L: 2.0±0.5, P=NS). PETCO2 values might account for some of these changes but this needs more exploration. While more data is necessary to fully interpret the findings, our results indicate that phase of the cycle may affect cerebrovascular tone and ability to maintain cerebral blood flow when upright. In addition, the impairment in autoregulatory response during the luteal phase may be related to increasing progesterone levels.