To evaluate the vascular effects of topically applied bradykinin on the human nasal mucosa, 13 asymptomatic hay fever patients and 11 non-allergic subjects were challenged with diluent or bradykinin in three increasing doses. Mucosal blood flow was determined with the 133Xenon wash-out method and expiratory peak flow measurements used to assess nasal airway resistance before and after challenge. Nasal symptoms were recorded. Nasal secretion quantity was measured from preweighed paper handkerchiefs. Bradykinin induced a slight increase in nasal airway resistance which was similar in both allergic and non-allergic subjects. Nasal secretion was clearly increased after challenge with bradykinin compared with challenge with diluent in both allergic and non-allergic subjects. Bradykinin did not, however, induce any change in mucosal blood flow in either group. The present findings could be explained by direct effects of bradykinin on the vascular bed without reflex activity. Bradykinin would then induce an increase in vascular permeability with subsequent oedema formation and increased amounts of fluid on the mucosal surface. In contrast to allergen challenge, bradykinin challenge had no effect on the resistance vessels, changes of which had previously been shown to be largely reflex-mediated.