Normal canine lungs were prepared for ultrastructural studies using two different routes for fixation: the airways route and the vascular route. Using the airways route, under conditions of controlled pressure, scanning and transmission electron microscopic studies of alveolar surfaces allowed identification of an average of 19 alveolar pores per exposed alveolar surface, each pore averaging approximately 3 micron. in diameter. The alveolar surface was honeycombed in appearance, with the walls of multiple capillaries bulging into the alveolar space. Using the other route, vascular perfusion of the lungs with the fixative and controlling pressures in both the vascular and airway compartments, scanning electron microscopic studies showed that most exposed alveoli exhibited a smooth or slightly wrinkled surface, essentially devoid of pores and averaging only three pores per alveolus. By transmission electron microscopy, alveolar surfaces were found to be covered with an extracellular material suggestive of lung surfactant; alveolar pores, averaging approximately 1 micron. in diameter, were filled with the same material. It is concluded that most alveolar pores of normal dogs are bridged by and filled with lung surfactant. These findings can be demonstrated most reliably by using vascular perfusion of the lungs for introducing the fixative.