It was shown previously that ATP acting via P2X receptors that contain the P2X₂ subunit, with or without P2X₃ subunit, contributes in a significant manner to the transmission of the sensitivity of the carotid body to changes in arterial PO₂ (Zhang et al. 2000; Prasad et al. 2001; Rong et al. 2003). In the present study we used mice with selective deletion of genes encoding P2X₂ (P2X₂-/-), P2X₃ (P2X₃-/-) or both subunits (P2X₂/P2X₃Dbl-/-) to determine whether in the carotid body ATP also mediates CO₂ chemosensory transduction. All experiments were carried out in adult (4-6 months) knockout and wild-type mice. Animals were killed by an overdose of pentabarbitone sodium (200 mg kg^-1). The carotid body and the attached sinus nerve were dissected. The sinus nerve was recorded in vitro using a suction electrode. The analogue of hypercapnia was induced by perfusing the recording chamber with aCSF in which sufficient extra CO₂ had been bubbled to reduce the pH from its normal value of 7.4 to a 7.0. It was found that (i) carotid sinus nerve resting discharge was reduced by 32% (n=9; p<0.05, Student's t test), 54% (n=9; p<0.05), and 83% (n=7; p<0.05), in P2X₂-/-, P2X₃-/- and P2X₂/P2X₃Dbl-/- mice, respectively; (ii) peak response to hypercapnia in P2X₂-/-, P2X₃-/- and P2X₂/P2X₃Dbl-/- mice was smaller than in their wild-type littermates (n=18) by 63% (n=9; p<0.05), 63% (n=9; p<0.05), and 86% (n=7; p<0.05), respectively; (iii) in preparations taken from the wild-type mice (n=8) P2X receptor antagonist 2′-(or-3′)-O-(trinitrophenyl)-adenosine 5′-triphosphate (TNP-ATP; 10 μM) reduced the baseline firing by 58% (p<0.05, Student's paired t test) and the peak response to hypercapnia by 71% (p<0.05); (iv) in preparations taken from the P2X₂-/- mice (n=5) TNP-ATP reduced the baseline firing by 76% (p<0.05) and the peak response to hypercapnia by 50% (p<0.05). From these data we conclude that in the carotid body ATP is partially responsible for the maintenance of the baseline discharge and mediates the effect of CO₂ on the activity of the sinus nerve afferent fibres. This study provides further evidence that ATP, acting via P2X receptors that contain the P2X₂ and P2X₃ subunits, plays a pivotal role in the carotid body function.