Fluctuations in alveolar CO2 and in base excess during the menstrual cycle

End-tidal PCO2 and base excess were measured on 10 healthy females throughout a total of 16 menstrual cycles. A corresponding decrease was observed in both variables during the luteal phase of the cycle. The curve for PCO2 was essentially that reported by previous authors, the tension dropping from a mean value of 39.8 to 36.7, 10 days after ovulation. The parallel drop in BE lowered the variable from -0.5 to -2.6. The resulting arterial pH throughout the cycle was calculated to be between 7.38 and 7.40. Since no time lag could be observed between the change in PCO2 and that of base excess, the primary disturbance could be either a ventilatory or metabolic response to an increase in progesterone during the luteal phase. In either case, a considerable decrease in body CO2 stores must result from these changes.