We previously showed that medroxyprogesterone acetate (MPA) effectively decreases the arterial CO(2) levels in postmenopausal women with partial upper airway obstruction. The aim of the present study was to analyze the effects of MPA on the inspiratory flow shapes during sleep. Eight postmenopausal women with hypoxemia and partial upper airway obstruction during sleep (patients) received MPA 60 mg daily for 14 days. Four matched postmenopausal women without MPA treatment served as controls. Sleep and nasal pressure were recorded on each visit. Each breath was analyzed for duration, volume and inspiratory flow shape class. MPA shortened inspiration and prolonged expiration. The inspiratory volumes increased consistently in all flow shape classes. The inspiratory shapes with single late peak were transformed to those with double peak. MPA also decreased shapes with mid-peak or mid-plateau. MPA did not have an effect on sleep. Sleep modified the flow shape distribution only in patients but in a similar fashion in stages S2, SWS and REM. The results suggest that postmenopausal women present with a significant proportion breaths with poor initial inspiratory flow, which is reversed with MPA-induced respiratory stimulation.