Nitric oxide (NO) is endogenously generated from two constitutively expressed nitric oxide synthase (NOS) isoforms, i.e., neuronal (NOS-1) and endothelial (NOS-3). Both isoforms are localized within the carotid body. Previous studies have shown endogenously generated NO modulates carotid body activity. In the present study, we examined the relative contribution of NO generated by NOS-1 and NOS-3 in respiratory reflexes arising from the carotid body. Experiments were performed on mutant mice deficient in NOS-1 or NOS-3. Wild-type (WT) mice, which contained both isoforms, served as controls. Respiration was monitored in unanesthetized mice by plethysmography. In anaesthetized mice, efferent phrenic nerve activity was monitored as index of breathing. We examined the effects of hypoxia (12% O2), cyanide and brief hyperoxia (Dejour's test) on respiration. In NOS-1 mutant mice, the ventilatory response to hypoxia (12% O2) were significantly augmented, compared to wild-type (WT) mice. By contrast, NOS-3 mutant mice displayed significantly blunted respiratory responses to hypoxia compared to WT controls. The responses to cyanide were augmented in NOS-1; whereas they were blunted in NOS-3 mutant mice. Respiratory depression in response to brief hyperoxia was more pronounced in NOS-1, while it was nearly absent in NOS-3 mutant mice. These results demonstrate that NO produced by the neuronal and endothelial NOS isoforms have different modulatory roles in carotid body chemosensitivity.