knowledge of liquid secretion by fetal lung stems from studies of sheep. the authors extended these studies to dogs and examined the persistence of the fetal pattern of airway epithelial permeability and ion transport in the neonatal animal. plasma and lung liquid from fetal dogs were analyzed for na/sup +/, k/sup +/, cl/sup -/ and hco/sub 3//sup -/. only the cl/sup -/ concentration of fetal lung liquid (129 meq/l) was significantly different from that of fetal plasma (111 meq/l). segments of trachea from fetal and neonatal (<1, 7-10, and 21-46 days after birth) dogs were excised and mounted in flux chambers. the transepithelial potential difference (pd) of all tissues was oriented lumen negative (9.8-14.8 mv). under short-circuit conditions, undirectional na/sup +/ flows were symmetrical. cl/sup -/ was secreted, and the secretion was equivalent to short circuit current (i/sub sc/). it is concluded that cl/sup -/ secretion by epithelium of large airways may contribute to fetal liquid lung production, but it is unlikely that the tracheal epithelium is involved in fluid absorption at birth whereas fetal bronchi appear to secrete cl/sup -/, neonatal bronchi absorb na/sup +/. this transition appears to be too slow to assist with rapid lung liquid clearance at birth but may help maintain liquid balance across the bronchial epithelium of the air-filled lung in neonates >2 wk of age