Experiments were performed on seven anesthetized fetal sheep to evaluate involvement of metabolites of the cyclooxygenase system in lung liquid production. Rates of lung liquid production were determined by dye dilution technique using blue dextran. The control rate of formation of lung liquid in these preparations was 4.6 ± 0.55 mL∙kg−1∙h−1. Administration of indomethacin (1.85 mg/kg) intraarterially to fetuses resulted in a 60% decrease in the rate of formation of lung liquid. The present studies suggest an involvement of products of the cyclooxygenase system in controlling the rate of secretion of lung liquid in fetal animals.