Slices of lungs from late-fetal (1 day pre-partum) and adult rats lost K+ and gained Na+, Cl-, water and Ca2+ during pre-incubation at 1 degrees C. These changes were reversed upon restoration to 37 degrees C. The recovery of composition at 37 degrees C was completely dependent on cell respiration in adult slices; by contrast, glycolysis could support partial recovery in the fetal slices. Ouabain completely inhibited K+ reaccumulation at both ages but inhibited net extrusion of water by no more than 50%. Replacement of medium Cl- with NO3- prevented the extrusion of water in the presence of ouabain in adult but not fetal slices. Transmission electron microscopy of type II epithelial cells in slices of both ages showed that ouabain induced the formation of many cytoplasmic vesicles, apparently derived from the Golgi apparatus. Regulation of cell ionic and water content is thus generally similar in late-fetal and adult lung tissue, but there are differences in the source of ATP and in some features of ouabain-resistant volume regulation.