Removal of fetal lung fluid at birth is crucial to survival. In vivo, a reversal in the direction of vectorial, amiloride-sensitive Na+ transport can be stimulated by ETYA, a nonmetabolizable analogue of the naturally occurring unsaturated fatty acid, arachidonate. Using the patch-clamp technique, fetal guinea pig alveolar type ll pneumocyte single Na+ channel activity was robustly activated by 10 muM arachidonate, ETYA, oleate and stearate; this was unaffected by cyclooxygenase and 5'lipoxygenase inhibitors. The Na+ channel expressed in fetal guinea pig alveolar epithelial type II pneumocytes has biophysical properties compatible with species-specific coexpression of a novel variant of alpha ENaC with beta ENaC. gamma ENaC is either not expressed in this tissue or shares very little homology with the rat and human gamma subunit. Thus, dramatic stimulation of this channel by arachidonate explains the in vivo observation of gestation-dependent reversal of fetal transepithelial driving force and may, therefore, be of physiological significance during the transition to breathing air at birth.