By reanalyzing our previous study (TAKANO et al., 1981) of resting ventilation and PACO2 during the menstrual cycle of eight healthy women, we found that relative to the follicular phase, a 9.4% increase in VA (p less than 0.05) and a 2.5 mmHg decrease in PACO2 (p less than 0.001) occurring during the luteal phase, both of which have been reported previously, were attributable mostly to a decrease in VD (-7.2%, 0.05 less than p less than 0.1) but not to an increase in VE (+1.9%, N.S.), the latter two changes producing an 8.7% decrease in VD/VT (p less than 0.01).