Peripheral chemoreflex sensitivity is potentiated in both clinical and experimental chronic heart failure (CHF). Downregulation of nitric oxide (NO) synthase in the carotid body (CB) is involved in this effect. However, it is unknown whether carbon monoxide (CO) also contributes to the altered peripheral chemoreflex sensitivity in CHF. This work highlights the effect of NO and CO on renal sympathetic nerve activity (RSNA) in response to graded hypoxia in conscious rabbits. RSNA responses to graded hypoxia were enhanced in CHF rabbits compared with that in sham rabbits. Both the NO donor (S-nitroso-N-acetylpenicillamine, SNAP, 1.2 µg/kg/min) and CO releasing molecule (tricarbonyldichlororuthenium (II) dimmer, [Ru(CO)₃Cl₂]₂, 3.0 µg/kg/min) each attenuated hypoxia-induced increases of RSNA in CHF rabbits (P<0.05), but the degree of attenuation of RSNA by either SNAP or [Ru(CO)₃Cl₂]₂ was smaller than that by SNAP plus [Ru(CO)₃Cl₂]₂. Conversely, combined treatment with the NO synthase (NOS) inhibitor (N-nitro-L-arginine, LNNA, 30mg/kg) and the heme oxygenase (HO) inhibitor (Cr (III) mesoporphyrin IX chloride, CrMP, 0.5 mg/kg) augmented the RSNA response to hypoxia in sham rabbits to a greater extent than either inhibitor alone, and was without effect in CHF rabbits. Using immunostaining and western blot analyses, we found expression of neural NOS (0.19±0.04, the ratio with &#946;-tubulin protein), endothelial NOS (0.17±0.06), and HO-2 (0.15±0.02) protein in CBs from CHF lower than that (0.63±0.04, 0.56±0.06 and 0.27±0.03, respectively, p<0.05) from sham rabbits. These results suggest that a deficiency of both NO and CO in the CBs augments peripheral chemoreflex sensitivity to hypoxia in CHF.