In this study, we applied time- and frequency-domain signal processing techniques to the analysis of respiratory and arterial O₂ saturation (Sa_O2 ) oscillations during nonapneic periodic breathing (PB) in 37 supine awake chronic heart failure patients. O₂ was administered to eight of them at 3 l/min. Instantaneous tidal volume and instantaneous minute ventilation (IMV) signals were obtained from the lung volume signal. The main objectives were to verify 1) whether the timing relationship between IMV and Sa_O2 was consistent with modeling predictions derived from the instability hypothesis of PB and 2) whether O₂ administration, by decreasing loop gain and increasing O₂ stores, would have increased system stability reducing or abolishing the ventilatory oscillation. PB was centered around 0.021 Hz, whereas respiratory rate was centered around 0.33 Hz and was almost stable between hyperventilation and hypopnea. The average phase shift between IMV and Sa_O2 at the PB frequency was 205° (95% confidence interval 198–212°). In 12 of 37 patients in whom we measured the pure circulatory delay, the predicted lung-to-ear delay was 28.8 ± 5.2 s and the corresponding observed delay was 30.9 ± 8.8 s ( P = 0.13). In seven of eight patients, O₂ administration abolished PB (in the eighth patient, Sa_O2 did not increase). These results show a remarkable consistency between theoretical expectations derived from the instability hypothesis and experimental observations and clearly indicate that a condition of loss of stability in the chemical feedback control of ventilation might play a determinant role in the genesis of PB in awake chronic heart failure patients.