We assessed ventilatory responses of 8 women to chronic contraceptive pill administration. We also studied 5 non-pill-taking women to determine if our techniques were capable of detecting the known ventilatory changes in the luteal phase of the menstrual cycle. During the high-progestin phase, relative to the low-progestin phase, the pill group had a mean increase in slope of CO2 response of 0.78 litres/min/mm Hg, a mean decrease in resting PETCO2 of 1.3 mm Hg, and no change in hypoxic sensitivity. The non-pill group had no change in slope of CO2 response, a mean decrease in resting PetCO2 of 1.1 mm Hg, and no change in hypoxic sensitivity. We conclude that the contraceptive pills studied cause a slight hyperventilation that is probably mediated by a central mechanism, as no changes in peripheral response were detected.