The aim of the research described in this thesis is to get more insight into various aspects of bronchial responsiveness. The literature is reviewed and discussed in the first part of this thesis (chapters 2-5) and emphasis is placed on recent developments. In the review results from studies in animals and in humans are correlated as much as possible. Our own studies are designed to analyse various aspects of the mechanisms in bronchial responsiveness in children, as well as the role of bronchial responsiveness in the occurrence of symptoms. These studies, started in 1976, are divided into three groups: a. Dose- and time-response relationships using inhaled histamine. These are analysed and the methods for the measurement of bronchial responsiveness considered (chapter 6). b. Studies on mechanisms of: 1. The relative contribution of the autonomic and the mast cell system in the pathogenesis of bronchial responsiveness using exercise is studied by means of protective agents (paragraph 7.1). 2. The pattern of bronchial obstruction after exercise is evaluated with flowvolume curves using air and helium/oxygen mixtures (paragraph 7.2). 3. The feasibility of leucocytes as a model for hyperresponsiveness at a cellular level are performed by measuring the release of mediators (paragraphs 7.3 and 7.4). c. The clinical significance of the relationship between bronchial responsiveness and the occurrence of bronchial obstruction after contact with allergens and after exercise (chapter 8).