Background: Slc26a9 is a Slc26 anion transporter family member with high expression in lung and stomach. Adult Slc26a9-/- mice loose the ability to secrete gastric acid (1), and do not upregulate a bronchial anion conductance during allergic asthma induction, thus developing bronchial mucus plugging (2). A striking finding during the mouse breeding was a strong excess mortality in the perinatal period in the Slc26a9 KO pups. Aim: To learn more about Slc26a9 expression and function in the newborn lung, and to search for the reason for excess perinatal mortality. Methods: The dead pups were collected, genotyped, and examined histologically. Slc26a9, CFTR and TMEM16a mRNA expression was measured by qPCR in microdissected proximal and distal airways of newborn and adult lung. Slc26a9-/-, CFTR-/-, and WT neonatal and adult trachea were studied electrophysiologically in Ussing chambers. Results: Dead pups found in the cages were genotyped and found to be Slc26a9-/- in >95%. Histology revealed mucus and cell debris in the trachea. Volume density measurements revealed a strong excess of tracheal and bronchial mucus in the Slc26a-/- pups (n= 7 KO, 8 WT). Slc26a9 mRNA expression was significantly higher in the distal than proximal airways, and significantly higher in neonatal than adult mice. CFTR mRNA expression levels followed a similar pattern, but not as pronounced as for Slc26a9. The amiloride-insensitive, bumetanide-sensitive short circuit current (Isc) was higher in neonatal Slc26-/- than WT trachea, but lower in neonatal CFTR-/- than WT trachea. Surprisingly, forskolin (FSK) + IBMX elicited a significantly higher Isc response (delta Isc) in neonatal Slc26a9-/- than WT trachea. This difference was abolished by preincubation with the CFTR inhibitor CFTRinh172. Neonatal CFTR-/- trachea displayed a FSK+IBMX stimulated delta Isc that was not different from WT, and a significantly stronger UTP-induced delta Isc. Adult Slc26a9-/- trachea did not display significant differences in the aforementioned parameters, but had a significant downregulation of both CFTR mRNA and TMEM16A mRNA. Summary and Conclusions: The anion conductance Slc26a9 is expressed at higher levels in neonatal than adult murine airways. Contrary to expectations, the bumetanide-sensitive Isc, and the FSK+IMBX activated delta sc, were higher in Slc26a9-deficient trachea and this difference was abolished by CFTR inhibition. Thus, the question arises whether the early death of Slc26a9-/- pups may be related to problems with clearing of airway liquid (and the mucus) rather than a lack of anion secretion. Adult airways do not show this phenotype, but display downregula-tion of both CFTR and TMEM16a expression in the large airways, possibly as a counterregulatory measure.