segments of fetal and maternal trachea, maternal bronchi from near-term sheep, and trachea and bronchi from nonpregnant adult sheep were excised and mounted as sheets in ussing chambers. the conductance (g) for each group of tissues was similar (=4 ms/cm/sup -2/): the short circuit current (i/sub sc/) ranged from 45-90 mua/cm/sup -2/. under short-circuit or open-circuit conditions trachea and bronchi from pregnant and nonpregnant adult animals absorbed na/sup +/, whereas fetal trachea secreted cl/sup -/ short-circuited maternal bronchi secreted k/sup +/, whereas maternal and fetal trachea did not. it is concluded that ion transport in fetal large airways contributes to the cl/sup -/ and liquid secretion by the entire fetal pulmonary epithelium, whereas resting ion transport of large airways from adult sheep, like that of mature airways of many species, is dominated by na/sup +/ absorption. all of these airway epithelia are characterized by large paracellular aqueous paths