Mucus secretion is a normal function of respiratory mucous membranes. Models for measurement of mucus production by cultured human and feline bronchial and nasal mucosae have been developed in order to examine the controls of mucus secretion. Corticosteroids inhibit MGP release by lowering baseline secretion. Analysis of corticosteroid treated airways reveals a close correlation between lipocortin generation and MGP production. Pulmonary inflammation with neutrophils is often associated with mucus production. Lysates of human neutrophils as well as supernatants from activated neutrophils cause airways to release MGP; this activity is not due to elastase, and identification of the mucus secretagogue is under study. Endorphins cause increased MGP secretion. Substance P is found in close association with submucous glands. Substance P causes MGP secretion, and the activity of substance P is markedly increased by protection from degradation by proteases with thiorphan. Platelet activating factor (PAF) activates feline airways to secrete MGP, and this action requires the formation of arachinoids and is inhibitable by CCS.