Intralaryngeal ammonia induces an airway protective reflex in rats characterized by apnea, bradycardia and increased laryngeal resistance. Since ammonia is known to stimulate both the myelinated and the nonmyelinated afferents, the role of nonmyelinated afferents in eliciting the above response is unclear. The present study was designed to investigate this by utilizing two techniques that selectively block C-fiber afferents: intravenous ruthenium red and perineural application of capsaicin. Ammonia vapor was introduced into the functionally isolated larynx of anesthetized rats for 10 sec at a flow rate of 5 ml/sec. Changes in expiratory duration were expressed as the ratio between test and control expiratory time (TEmax/control). Perineural application of capsaicin (n = 8) to SLNs attenuated the responses of both intralaryngeal ammonia (TEmax/control: 12.32 +/- 1.67 to 6.89 +/- 1.62; P < 0.05) and capsaicin (8.86 +/- 2.02 to 3.35 +/- 2.17; P < 0.05). Pretreatment with ruthenium red (1 mg/kg, iv., n = 10) significantly reduced the effects of ammonia (20.43 +/- 4.38 vs. 11.66 +/- 3.21; P < 0.05) and nebulized capsaicin (16.28 +/- 4.58 to 3.90 +/- 0.83; P < 0.05). These results suggest that the C-fiber endings of the SLN play an important role in eliciting the airway protective reflexes by irritants such as ammonia.