At a late stage of fetal development, the mammalian alveolar epithelium undergoes an abrupt differentiation as a part of the preparation of the lung for the postnatal demands of gas exchange. Some of the most striking changes occur in the type II pneumocytes as they lose their glycogen and start to produce the lamellated inclusion granules that contain pulmonary surfactant. Premature birth before adequate type II cell maturation results in the neonatal respiratory distress syndrome, which is frequently fatal. We have used serial ultrathin sectioning, electron microscopy, and three-dimensional reconstructions to study the ultrastructural features of maturation of rat type II cells from a single rat each at age gestational day 20 through adult stages. We found evidence over this time span for compartmentation of several secretory granule precursors within type II cells. Changes in the polarization of lamellar bodies were observed over the time period studied. We also found marked gestational changes in the number and morphology of type II cell cytoplasmic processes that perforate the basement membrane. Type II cell mitochondria changed in shape during postnatal development from single, spherical to complex, branched structures. Volume composition obtained from serial sections of a small number of type II cells agreed closely with published morphometric data, indicating that throughout the animal's lifespan, type II cells are a homogenous population.