Morphologic, physiological, and biochemical changes occur in the carotid body (CB) during postnatal development in relation to physiological requirements. Chronic normobaric hyperoxia attenuates the carotid chemosensory response to hypoxia. During aging there is less of a CB response to hypoxia, which results in a reduced ventilatory adaptation and chemosensory discharge. To test if the oxygen-sensitive mechanism is affected by chronic hyperoxia in an age-dependent fashion, we have studied structural and ultrastructural aspects of young and old rat CBs. Four groups of six male Wistar rats were used. One group of two-month-old rats and another of 25-month-old rats were kept at room air. The other two groups, age matched, were exposed to 98-100% O2, for 60-65 h, in a large Plexiglas chamber. The rats were anesthetized, CBs were fixed in situ with glutaraldehyde (2.5% in phosphate buffer. pH 7.4, 320 mOsm), and were prepared for electron microscopy. Young hyperoxic rats showed focal necrosis in type I cells, along with an increase of endoplasmic reticulum, Golgi apparatus, and of mitochondria volume, with loss of cristae. These changes were less pronounced in the older rat CBs compared with the young rats. In conclusion, hyperoxia seems to affect the oxygen-sensitive mechanism in the carotid body cells, and the reduced effects shown in the old rat CBs suggest an age-related decreased sensitivity to oxygen.