1. Twenty-two newborn infants with early respiratory distress syndrome (all but one cyanotic in room air) were studied. 2. By the hyperoxia test (100% O2 during 30 minutes) differences in behavior of the expected (though unlocalized) true right to left shunt, during the first day of life, allowed separation into three patterns, designated as Types I, II, and III. 3. Type I (eleven infants) was characterized by a large right to left shunt at 4 hours, which increased further during the next 24 hours. Functional residual capacity was decreased; tidal volume to functional residual capacity (VT/FRC) ratio was normal or high. Mixed acidosis, present at 4 hours, remained unchanged during the next 24 hours. Eight infants died with atelectasis associated seven times with hyaline membranes, as shown by autopsy. 4. Type II (eight) infants had a right to left shunt within the normal range during the first day. Functional residual capacity was normal, with low VT/FRC ratio. The important mixed acidosis in infants of this type at four hours disappeared later as the lung condition improved. None died. It is suggested that this respiratory distress was related to aspiration. 5. Type III (three) infants showed a large right to left shunt between 1 and 4 hours of age, with rapid normalization in the next 24 hours. There was no evident association between this type of respiratory distress and a particular disease state. 6. Type I infants were treated with sodium bicarbonate solution, with no effect on CO2 retention or right to left shunt. Seven of the ten treated infants died. 7. The necessity of distinguishing between different types of the respiratory distress syndrome, and the importance of massive oxygen therapy in the type associated with hyaline membranes, became obvious during this study.