The involvement of tachykinins in the airway reactivity of ovalbumin (OA)-sensitized rats was studied by capsaicin (CAPS) treatment. In subcutaneously sensitized animals, the reactivity to both OA aerosol and serotonin given intravenously was decreased when CAPS was given after the sensitization period. No effects on the serum IgE and IgG antibody levels were seen in these animals. In contrast, when CAPS was given before the sensitization period, no effects were seen on the OA aerosol and serotonin reactivities. Immunohistochemical examination revealed that the la antigen expression in the bronchial epithelium was increased by both subcutaneous and aerosol sensitization. The CAPS treatment decreased this la antigen expression. Histological examination of mononuclear cells, mast cells and goblet cells revealed only small effects on the cell numbers by both the OA sensitization and the CAPS treatment. The results demonstrate a link between tachykinins, serotonin, the immune system and clinical lung reactivity. The mechanisms for this seem to be complex, since the timing of the CAPS treatment and antigen sensitization is crucial for the outcome.