In the past decade or two it has become apparent that disorders of the respiratory system represent the single most important cause of death of the newborn infant. The work on experimental animals by Barcroft and his coworkers, and subsequently by Dawes and his associates, has done much to clarify the critical physiologic adjustments of the cardiopulmonary system at the time of birth. Cross has extended physiologic investigations to the human newborn infant. In recent years a number of investigators have applied similar techniques to the study of abnormal neonatal respiratory adaptions.