Hypoxia increases the release of neurotransmitters from chemoreceptor cells of the carotid body (CB) and the activity in the carotid sinus nerve (CSN) sensory fibers, elevating ventilatory drive. According to previous reports perinatal hyperoxia causes CSN hypotrophy, and varied diminishment of CB function and the hypoxic ventilatory response. Present study aims to characterize the presumptive hyperoxic damage. Hyperoxic rats were born and reared for 28 days in 55%-60% O₂; subsequent growth (to 3.5-4.5 months) was in a normal atmosphere. Hyperoxic and control rats (born and reared in normal atmosphere) responded with a similar increase in ventilatory frequency to hypoxia and hypercapnia. In comparison to control, hyperoxic CBs showed: 1) half the size, but comparable percentage area positive to tyrosine hydroxylase (chemoreceptor cells) in histological sections; 2) a 2-fold increase dopamine (DA) concentration, but a 50% reduction in DA synthesis rate; 3) a 75% reduction in hypoxia-evoked DA release, but normal high [K⁺]_e-evoked release; 4) a 75% reduction in the number of hypoxia- sensitive CSN fibers; however, responding units displayed a nearly normal hypoxic response; and 5) a smaller percentage of chemoreceptor cells which increased [Ca²⁺]_i in hypoxia, although responses were within the normal range. We conclude that perinatal hyperoxia causes atrophy of the CB/CSN complex, resulting in smaller number of chemoreceptor cells and fibers. Additionally, hyperoxia damages O₂-sensing, but not exocytotic, machinery in most surviving chemoreceptor cells. Although hyperoxic CBs contain substantially smaller numbers of chemoreceptor cells/sensory fibers responsive to hypoxia, they appear sufficient to evoke normal increases in ventilatory frequency.