In anesthetized rats, vagal afferent activities activate slow central mechanisms which modulate the pattern of breathing over several breaths, giving rise to increased breath to breath variability of respiratory pattern. We hypothesized that variability in breathing pattern would produce variability in blood gases and further enhance breath to breath variability of inspired ventilation. Anesthetized rats were placed in a head-out plethysmograph and spontaneous breathing recorded during inhalation of room air and 100% oxygen. The standard deviations and coefficients of variation of ventilation were similar for both inspired gases, but the shapes of the power spectra of ventilation differed, indicating a relative increase in low-frequency power on room air in those animals exhibiting little low-frequency power on oxygen. Simple indices of variability cannot discriminate these temporal changes in breathing pattern variability.