Abstract Rats reared in hyperoxia have smaller carotid bodies as adults. To study the time course and mechanisms underlying these changes, rats were reared in 60% O2 from birth and their carotid bodies were harvested at various postnatal ages (P0–P7, P14). The carotid bodies of hyperoxia-reared rats were smaller than those of age-matched controls beginning at P4. In contrast, 7d of 60% O2 had no effect on carotid body size in rats exposed to hyperoxia as adults. Bromodeoxyuridine (BrdU) and TdT-mediated dUTP nick end labeling (TUNEL) were used to assess cell proliferation and DNA fragmentation at P2, P4, and P6. Hyperoxia reduced the proportion of glomus cells undergoing cell division at P4; although a similar trend was evident at P2, hyperoxia no longer affected cell proliferation by P6. The proportion of TUNEL-positive glomus cells was modestly increased by hyperoxia. We did not detect changes in mRNA expression for proapoptotic (Bax) or antiapoptotic (Bcl-XL) genes or transcription factors that regulate cell cycle checkpoints (p53 or p21), although mRNA levels for cyclin B1 and cyclin B2 were reduced. Collectively, these data indicate that hyperoxia primarily attenuates postnatal growth of the carotid body by inhibiting glomus cell proliferation during the first few days of exposure.