We examined the effects of acute exposure to ozone on the permeability of the tracheal mucosa and the contribution of neural pathways to the effects of ozone using horseradish peroxidase (HRP; mol wt 40,000) as a marker of lumen-to-blood transfer of a macromolecule in guinea pigs in vivo. Each guinea pig was anesthetized and exposed for 30 min to either ozone [0.5 or 3 parts/million (ppm)] or air. Immediately after exposure, a tracheal segment was isolated between two polyethylene cannulas in vivo and filled with HRP solution (50 mg/ml). Blood samples were drawn before and 10, 20, 30, and 40 min after the intratracheal instillation of HRP. The plasma levels of HRP in guinea pigs exposed for 30 min to 3 ppm of ozone, but not to 0.5 ppm of ozone, were significantly greater than those in guinea pigs exposed to air. Although the increased plasma HRP levels after exposure to 3 ppm of ozone were unaffected by propranolol or atropine, they were completely inhibited by pretreatment with capsaicin (50 mg/kg sc, injected in two doses). These results suggest that endogenous neuropeptides mediate the ozone-induced increase in the permeability of the tracheal mucosa in guinea pigs in vivo, but neither an adrenergic nor a cholinergic pathway appears to be involved.