Caveolin-1 is a scaffolding protein component of caveolae, membrane invaginations involved in endocytosis, signal transduction, trans- and intracellular trafficking, and protein sorting. In adult lung, caveolae and caveolin-1 are present in alveolar endothelium and Type I epithelial cells but rarely in Type II cells. We have analyzed patterns of caveolin-1 expression during mouse lung development. Two caveolin-1 mRNAs, full-length and a 5′ variant that will translate mainly into caveolin-1 α and -β isoforms, are detected by RT-PCR at embryonic day 12 (E12) and afterwards in the developing and adult lung. Immunostaining analysis, starting at E10, shows caveolin-1α localized in primitive blood vessels of the forming lung, in an overlapping pattern to the endothelial marker PECAM-1, and later in all blood vessels. Caveolin-1 α is not detected in fetal or neonatal lung epithelium but is detected in adult epithelial Type I cells. Caveolin-1 was previously shown to be expressed in alveolar Type I cells. These data suggest that expression of caveolin-1 isoforms is differentially regulated in endothelial and epithelial cells during lung development. Caveolin-1α is an early marker for lung vasculogenesis, primarily expressed in developing blood vessels. When the lung is fully differentiated postnatally, caveolin-1 α is also expressed in alveolar Type I cells.