Time course for changes in carotid body (CB) volume, CB O2 sensitivity, and the hypoxic ventilatory response (HVR) in neonatal rats reared from birth in 60% O2. Carotid body O2 sensitivity is based on the peak single-unit chemoreceptor activity during severe hypoxia (0% O2), while HVR represents the early phase of the response to 12% O2 (i.e., first minute of hypoxia). Values (mean ± SEM) are expressed as a percentage of those measured for age-matched rats reared in room air (i.e., “Control” rats); n=6 per age for CB volume, n= 6–16 per age for CB O2 sensitivity, and n=15–17 per age for HVR. Data were compiled from previously published studies (Donnelly et al., 2005; Bavis et al., 2010, 2011b; Dmitrieff et al., 2012).