In order to study the structure of membranes of tubular myelin and lamellar bodies (pulmonary surfactant), tissue from adult rat lung was prepared for freeze fracture by the rapid freezing method of Heuser et al. which requires no prior fixation or cryoprotection. Other tissues were freeze substituted and thin sectioned. A layer of tissue 50-100 mu thick was well preserved, but alveoli were partially collapsed. Freeze-fractured lamellar bodies were composed of tightly packed stacks of smooth lamellae about 100 A thick. Cross-fractures through tubular myelin exposed membranes organized into the square lattice described earlier. Fractures parallel to the longitudinal axis exposed particles arranged in rows which coincided with the corners. Membrane faces between the rows were smooth. These observations suggest (1) that in vivo the content of lamellar bodies is most likely arranged in layers, (2) that tubular myelin is present in tissues unexposed to fixatives or lipid solvents, (3) that smooth-surfaced lamellar body membranes become particulate when they form tubular myelin, and (4) that chemical fixation does not alter the general appearance of tubular myelin but it may affect lattice dimensions.