Ascaris suum hypersensitive dogs were exposed to ascaris aerosol, histamine, and acetylcholine at different time intervals separately to the upper part of the airtract and to the lower airways. Before and during the exposure the plasma histamine concentrations were measured. Acetylcholine and histamine delivered at the upper part of the airtract do not cause bronchoconstriction. Histamine diffuses in relatively high amounts through the mucosa of the upper part of the airtract into the arterial plasma. Ascaris extract causes bronchoconstriction from the upper part of the airtract as well as from the lower airways. Histamine possibly changes the diffusibility and the sensitivity of the histamine receptor. The correlation between Edyn during acetylcholine and histamine aerosol inhalation was statistically significant. The bronchoconstrictive reaction pattern caused by challenge of the upper part of the airtract is probably different from this caused by challenge of the lower airways.