Twenty near-term fetal goats, delivered by Caesarian section, under chloralose (50 mg/kg), with intact umbilical cords, were monitored for lung fluid production by a dye dilution method. Controls, and fetuses given slow infusions of saline (0.9% (w/v) NaCl at 0.028 - 0.042 ml/kg per min, intravenous, for 2 h), showed no significant increases in lung fluid production, Na+ secretion, or Na+ concentration in lung fluid. Intermediate rates of infusion (0.053 ml/kg per min for 2 h) did not increase fluid production, but significantly increased Na+ secretion and Na+ concentration in lung fluid. Fast infusions (0.084 - 0.24 ml/kg per min for 2-3 h) increased significantly fluid secretion (up to 40-fold), Na+ secretion (up to 9.3-fold), and Na+ concentration in lung fluid (increases up to 5.2 mequivalents/l); in one case, spontaneous reabsorption was reversed to secretion. Prolactin, placed directly in the lungs (10 micrograms/ml fluid), by the third hour; controls showed no changed. In all cases Cl- behaved similarly to Na+; K+ moved partly independently. Synthetic pentagastrin (infused at 0.12-2.5 micrograms/kg per min, intravenous) did not stimulate secretion. The results suggest that the fetal lungs may influence salt-water balance, and this may be under prolactin control.