Electron microscopy of the rabbit eustachian tube showed the presence of osmiophilic lamellar bodies in the cytoplasm of secretory cells and of osmiophilic free lamellar membranes in the lumen of tubal glands or above the epithelium. Similar structures have been reported in other parts of the respiratory system and in the pulmonary alveoli. The functional significance of these morphological observations is suggestive of a surface-tension-lowering effect on the mucosal lining layer of the eustachian tube and of a possible relationship to the phospholipid surfactant of the lung.