The surface epithelium of newborn ferret airways matures rapidly in the first month of life. Prominent developmental features include a transition from predominantly non-ciliated to ciliated cells, quantitative and qualitative changes in secretion of macromolecules, and a transition from secretory to absorptive patterns of ion transport. Freeze-fracture replicas of ferret tracheal epithelium from 0 to 28 days of age exhibited progressive developmental patterns in tight junctional structure from beaded, unclosed patterns in newborns to more closed patterns at 28 days. Strand number increased while the depth of tight junctional structures and the proportion of strands exhibiting discontinuity decreased postnatally. Total transepithelial conductance, paracellular conductance, and cell size decreased over the first month. Our data suggest that changes in physiological parameters that reflect epithelial tight junction permeability can be attributed, at least in part, to maturation of this intercellular junction during the postnatal period.