Information on the control and modulation of airway epithelial cells and their secretion is obtained by three techniques: 1) in vivo studies of animal models of disease, 2) in vitro studies by organ culture of human and animal model airways, and 3) chemical analysis of human and animal bronchial secretion. The contribution of each of these techniques is described in this paper, including recent or new information. In vivo models of mucous hypersecretion can be produced by irritants, infection, and drugs more quickly than previously expected. In the rat, beta 1 and beta 2 receptors are present with evidence of different activity in various airway regions. Organ culture studies combine autoradiographic analysis of cell activity with chemical analysis of secretory product, and describe inhibitory effect of new agents such as VIP. The application of density-gradient ultracentrifugation gives total recovery of undegraded macromolecules from bronchial mucus; it is now possible to recover mucous glycoprotein of molecular weight larger than that previously isolated. The organ culture studies and density-gradient ultracentrifugation studies indicate that a proteoglycan is a significant constituent of total bronchial secretion. Differences between diseases are emerging in the macromolecular partitioning between sol and gel obtained at 160,000 X g, a higher speed than that previously applied systematically in such studies.