Preterm fetal lambs of a gestational are highly susceptible to hyaline membrane disease were placed on apneic oxygenation while keeping the umbilical circulation intact and still connected to the mother ewe for the removal of metabolically produced CO2. After 4-6 hrs the total lung compliance and the chest x-ray films had markedly improved, allowing normal pulmonary ventilation with a mechanical ventilator. Fetuses older than 135 days gestation were later extubated and allowed to breathe room air unassisted. Fetuses 134 days gestation and younger were kept on mechanical ventilation. None of the animals so treated developed hyaline membrane disease. We believe pulmonary conditioning is an important tool in the prevention and the treatment of hyaline membrane disease. In this animal model we have shown that hyaline membrane disease is a preventable and curable disease.