AbstractMidkine (MK) is a 13‐kDa heparin‐binding growth factor that is thought to mediate developmental processes, including vasculogenesis, cell migration, and proliferation in various organs. To determine whether MK plays a role during lung morphogenesis, immunostaining for MK was assessed in mouse lung from embryonic day (E) 13 to postnatal day (PN) 24. MK was detected in mesenchymal and respiratory epithelial cells of the peripheral mouse lung from E13.0 to E15.5. From E18.5 to PN1, MK was observed primarily in epithelial cells lining conducting airways and peripheral lung saccules. By PN10, expression was no longer observed in respiratory epithelial cells but was readily detected in small blood vessels in the alveolar region of the lung. Although most respiratory epithelial cells uniformly expressed MK before E13.0, MK was restricted to subsets of cells by E18.5, colocalizing with the Clara cell secretory protein (CCSP) marker in conducting airways and with pro‐SPC, a marker specific for alveolar type II pneumocytes. By PN10, MK was not detected in respiratory epithelial cells of the conducting airways and was closely associated with capillary networks. The sites of intense MK staining in the respiratory epithelial cells correlated with sites of expression of thyroid transcription factor (TTF) ‐1, a transcription factor regulating formation and gene expression in the lung parenchyma. TTF‐1 enhanced transcription of the mouse MK gene promoter, acting on TTF‐1 regulatory elements located in the 5′‐region of the gene. Furthermore, MK expression was not detected in lungs of TTF‐1 null mice. TTF‐1 regulates expression of MK in the lung. The temporal/spatial distribution of midkine is consistent with a potential role in paracrine signaling during lung morphogenesis. Developmental Dynamics 227:227–237, 2003. © 2003 Wiley‐Liss, Inc.