Recent studies indicate that chemoafferent nerve fiber excitation in the rat carotid body is mediated by acetylcholine and ATP, acting at nicotinic cholinergic receptors and P2X₂ purinoceptors, respectively. We previously demonstrated that, after a 10- to 14-day exposure to chronic hypoxia (CH), the nicotinic cholinergic receptor blocker mecamylamine no longer inhibits rat carotid sinus nerve (CSN) activity evoked by an acute hypoxic challenge. The present experiments examined the effects of CH (9–16 days at 380 Torr) on the expression of P2X₂ purinoceptors in carotid body and chemoafferent neurons, as well as the effectiveness of P2X₂ receptor blocking drugs on CSN activity evoked by hypoxia. In the normal carotid body, immunocytochemical studies demonstrated a dense plexus of P2X₂-positive nerve fibers penetrating lobules of type I cells. In addition, type I cells were lightly stained, indicating P2X₂ receptor expression. After CH, the intensity of P2X₂ receptor immunostaining was maintained in chemosensory type I cells and in the soma of chemoafferent neurons. P2 receptor expression on type I cells was confirmed by demonstrations of ATP-evoked increased intracellular Ca²⁺; this response was modulated by simultaneous exposure to hypoxia. In normal preparations, CSN activity evoked by hypoxia in vitro was 65% inhibited in the presence of specific P2X₂ receptor antagonists. However, unlike the absence of mecamylamine action after CH, P2X₂ antagonists remained effective against hypoxia-evoked activity after CH. Our findings indicate that ATP acting at P2X₂ receptors contributes to adjusted chemoreceptor activity after CH, indicating a possible role for purinergic mechanisms in the adaptation of the carotid body in a chronic low-O₂ environment.