Twenty-one small premature infants with clinical features and radiological findings of type II RDS were studied. Correction of blood pH, corresponding with the decrease of blood Paco2, was usually seen in the first eight hours of age. Clinical improvement, as judged by the RDS score, was noted only after 12 hours of age. Complete recovery was the rule although signs of respiratory distress in a few infants lasted for 48–72 hours. The calculated AaDo2and total Qs/Qt were slightly higher (27.8%) than reported in normal infants and remained unchanged until after 48 hours of age when it decreased to 24.5%.