We studied the relationship between exercise hyperpnea (i.e., ventilatory dynamics) at the onset of exercise and hypercapnic ventilatory response (HCVR), and their differences between the follicular (FP) and luteal (LP) phases of the menstrual cycle in six healthy females. HCVR was tested under three O₂ conditions: hyperoxia (FiO₂=1.0), normoxia (0.21), and hypoxia (0.12). HCVR was defined as the relationship between the end-tidal   and minute ventilation ( ) using the regression line of the CO₂ slope and a mimetically apneic threshold of CO₂. HCVR provocation and measurements were conducted using an inspired CO₂ concentration of up to approximately 8mmHg higher than the end-tidal   level of basal isocapnic the end-tidal   at each menstrual both the slope and threshold in HCVR showed no statistically significant difference between LP and FP under any inspired FiO₂ conditions. In the case of exercise hyperpnea during the onset of submaximal exercise, the mean response time (MRT) in   dynamics showed no significant difference between LP and FP. Consequently, MRT in   response was not related to the slope in HCVR. During steady-state exercise, even though the   showed no significance between LP and FP,   was significantly related to the slope in HCVR ( ,  ). Exercise ventilation (i.e.,  ) would partly be adjusted by the enhancement of the chemoreflex drive to CO₂ only during the steady-state exercise.