Although guinea pigs have been frequently used as a model of asthma, antibodies produced in this species are generally gamma1 and gamma2 and belong to IgG. The antibody responsible for asthmatic attacks in humans is IgE, and such is quite different from gamma1 and gamma2, immunologically. Guinea pigs are not therefore an adequate model for investigating anti-asthmatic drugs which inhibit IgE-mediated mediator release, such as disodium cromoglycate. On the other hand, rats do produce an antibody similar to human IgE, the so-called homocytotropic antibody (HTA), by sensitization with dinitrophenylated ascaris extract (DNP-As) together with killed Bordetella pertussis as an adjuvant. To rats actively sensitized with DNP-As or passively sensitized with HTA serum against DNP-As, intravenous administration of antigen did not produce a transient increase in respiration (unlike that of guinea pigs) immediately after the antigen treatment, but a respiratory disorder similar to that seen during asthmatic attacks in humans did occur. The response to antigen was reproducible in passively sensitized rats compared with that of actively sensitized ones, though the symptom was moderate. The effect of N(3', 4'-dimethoxycinnamoyl) anthranilic acid (N-5'), a new anti-allergic drug, was determined in cases of experimental asthma in passively sensitized rats. Respiratory disorders as a result of antigen were clearly inhibited with oral administration of this agent.