We investigated the effect of ovarian hormones and aging on breathing pattern [pulmonary minute ventilation (V(E))], tidal volume (V(T)), breathing frequency (F(b)), and respiratory sinus arrhythmia (RSA) in women. Recordings of V(E) and electrocardiogram (ECG) were obtained from 23 healthy women (10 premenopausal, 13 postmenopausal) under resting, isocapnic hypoxia (IH), and euoxic hypercapnia (EH) conditions. Premenopausal women were tested on three different days, each day corresponding to a specific phase of the menstrual cycle (follicular, mid-cycle, and luteal); postmenopausal women (PMW) were tested on 1 day only. On each test day, subjects were challenged with IH and EH. The order of the two tests was randomized and separated by at least 1 hour. Due to the low F (b) of several PMW, the band limits for RSA analysis had to be adjusted. The spectral coherence between respiratory flow and ECG RR-interval was used to determine the spectral band. Within the spectral band, there was a consistent phase relationship between the two variables where high values of spectral coherence indicate a well-defined phase relationship between respiratory flow and RR-interval variability. The main findings in this study for RSA are fourfold. First, RSA did not change with different levels of ovarian hormones (progesterone, serum 17beta-estradiol) during the menstrual cycle. Second, RSA was not influenced by hormone replacement therapy. Third, RSA did not change with age. Fourth, RSA did not change with IH and EH-induced changes in breathing patterns. Finally, high individual variability of average RR-interval change per breath was found.