Mucous secretions covering the surface of the trachea and major bronchi of the rabbit lung were demonstrated with the scanning electron microscope. The surface secretion forms a continuous layer overlying the major airways and shows a complex 3-dimensional arrangement of overlapping sheets and fibrous networks. In the main and lobar bronchi, mucus forms more expanded networks, and in the trachea, mucus forms a smoother, more cohesive layer overlying the epithelial cells. The mucous blanket follows closely the contours of the airway epithelium and is situated at the tips of the cilia.