Hyperoxia in the immediate perinatal period, but not in adult life, is associated with a life-long impairment of the ventilatory response to acute hypoxia. This effect is attributed to a functional impairment of peripheral chemoreceptors, including a reduction in the number of chemoreceptor afferent fibers and a reduction in "whole nerve" afferent activity. The purpose of the present study was to assess the activity levels of single chemoreceptor units in the immediate post-hyperoxic period to determine if functional impairment extended to single chemoreceptor units and whether the impairment was only induced by hyperoxia exposure in the immediate post-natal period. Two groups of rat pups were exposed to 60% FiO₂ for two weeks at ages 0d-14d and 14d-28d at which time single unit activities were isolated and recorded, in vitro. Compared to control pups, hyperoxia-treated pups had a 10-fold reduction in baseline (normoxia) spiking activity. Peak unit responses to 12, 5 and 0% O₂ were reduced and nerve conduction time was significantly slower in both hyperoxia-treated groups compared to control groups. We conclude that: 1) hyperoxia greatly reduces single-unit chemoreceptor activities during normoxia and acute hypoxia, 2) the treatment effect is not limited to the immediate newborn period and 3) at least part of the impairment may be due to changes in the afferent axonal excitability.