There is considerable variability in the effect of histamine between species and between different sites within the airways. This is probably due to differences in the distribution of histamine receptors. In general, H1-receptors, which predominate in the airways of most species, mediate bronchoconstriction and H2-receptors mediate bronchodilation. In man, particularly in asthmatics, histamine is a powerful bronchoconstrictor, due to the predominance of bronchoconstricting H1-receptors in the airways. H1-receptor antagonists, given in adequate dosage, relieve bronchial tone in asthmatics, prevent histamine-induced bronchospasm in normal and asthmatic subjects and partially prevent both antigen- and exercise-induced asthma. The evidence concerning the presence and function of H2-receptors in human airways is contradictory, but, if present, their role is trivial. There is no difference in the pattern of histamine receptors in normal and asthmatic subjects.