To test the effect of nitric oxide synthase (NOS) blockade on the ventilatory responses to carotid body chemoreceptor stimulation in freely behaving animals, chronically instrumented adult Sprague-Dawley rats received increasing intravenous doses of sodium cyanide (NaCN; 0-300 micrograms kg-1) before and after i.v. administration of either 100 mg kg-1 N-nitro-L-arginine methyl ester (L-NAME), a non-specific NOS blocker, or 10 mg kg-1 S-methyl-L-thiocitrulline (SMTC), a selective neuronal NOS inhibitor. SMTC did not modify the NaCN dose-response curve. In contrast, L-NAME significantly enhanced the ventilatory responses to NaCN. Western blots of equivalent amounts of protein from carotid body tissue homogenates revealed higher levels of endothelial NOS than of neuronal NOS. We conclude that endothelial NOS provides the major source for NO within the carotid body, and exerts a down-regulatory effect upon peripheral chemoreceptor responsivity.