Premature rabbit fetuses were delivered by hysterotomy and, prior to the first breath, a concentrated homologous surfactant suspension was deposited in the pharynx. With repeated radiograms, the effect of this treatment was studied during the first two hours of extrauterine life. The lung aeration was quantitated with densitometry, and the chest expansion was assessed by measuring the inferior angle formed by the fourth ribs. In comparison with control animals, the treated animals demonstrated a greater chest expansion, and their lungs were significantly better aerated. Clinical application has to await further studies.