To determine the patho-physiological effects of heated vapour to the normal or allergic nasal mucosa, we measured the nasal resistance before and after a 10 min. exposure of hyperthermal (43.0 degrees C) aerosol to the nasal mucosa in normal subjects and perennial allergic rhinitis patients. In the allergic patients the mean nasal resistances after hyperthermal stimulation were significantly higher than those resistances without stimulation, both in expiration or inspiration. No significant differences of nasal resistances in normal individuals during the whole schedule with and without heated aerosol stimulation were found on expiration or inspiration. The local heated aerosol exposure increases the nasal resistance in nasal allergic patients while in normal subjects no changes were found, and the reaction may have arisen from a non-specific hypersensitivity of the susceptible allergic nasal mucosa.