Rabbit fetuses were injected intramuscularly with 0.5 mg. of isoxsuprine on the twenty-eighth day of gestation. They were killed in utero four hours after the injection, and fetal pulmonary fluid (FPF) was collected through a tracheal catheter. The quantity of FPF and the lung weight/body weight ratio were both significantly less in the isoxsuprine-treated fetuses than in control fetuses. Surface activity, evaluated with pulsating bubble, and the lecithin/sphingomyelin (L/S) ratio were greater in FPF from isoxsuprine-treated animals than in control samples. We concluded that isoxsuprine is able to dehydrate the fetal lung and cause a release of surfactant stored in type II pneumocytes. This latter conclusion was supported by a significant decrease in the number of lamellar inclusions observed in these cells.