At birth the epithelium of the distal airway tract switches from secretion to absorption. To investigate the role for water and ion transporters in this process, the mRNA expression of the water channels, aquaporins (AQP), Na,K-ATPase (NKA-α1) and amiloride sensitive Na+ channel (ENaC-α) were studied in fetal (E18 to E21) and postnatal (P2 to P40) rat lung. AQP1 mRNA levels increased sharply from E21 to P2, decreased transiently between P10 and P14 and reached maximal level at P40. AQP5 mRNA became detectable at P2, increased gradually from P18 and reached maximal level at P40. AQP2 and AQP3 mRNA were not detected in lung tissue. AQP4 mRNA was not detected in fetal lung tissue, increased sharply between E21 and P2 and decreased again between P2 and P10. The highest expression occurred at P2. The transient induction of AQP4 mRNA at birth was lung tissue specific and did not occur in brain and kidney, which also express AQP4. Immunocytochemistry revealed that AQP4 was located in the basolateral membranes of bronchial epithelial cells. Exposure of the fetal rats (E20) to ß-agonist or glucocorticoid hormone(GC) significantly increased lung AQP4 mRNA. The level of NKA-α1 and ENaC-α increased from E18 to birth. In summary, removal of lung liquid at birth is associated with pronounced changes in the expression of ion and water transporters. Three AQP (AQP1, 4, 5) have been identified in the lung with different ontogenic expression. The expression of AQP4 in bronchial epithelial cells suggest that AQP4 plays a role for absorption of lung fluid at birth.