It is well known that increasing tendency of emergency surgery of congenital snomalies such as tracheoesophageal fistu1a, esophageal atresia, esophageal hiatal hernia, hypertrophic pyloric stenosis,intestinal obstruction, imperforated anus, omphalocele, and meconium ileus by virtue of improvement of pediatric surgical procedure, anesthetic agent, and anesthetic method. The possibility of a low gastric pH and the resultant pulmonary damage if aspirated must be considered in the initial care of the newborn with poor muscle tone or reflex activity as well as in the anesthetic management of neonates. We became interested in the factors determining neonatal gastric pH in this point of view, and have measured gastric pH in 75 unselected neonates, 55 of whom were born vaginally and 20 by cesarean section. In premature infants, pH was high regardless of mode of delivery. In mature infants, pH was 1) significantly lower after vaginal delivery than after cesarean section; 2) tended to be lower after section preceded by labor than after elective section; and 3) was lowest after precipitate delivery. There was no correlation between neonatal gastric pH and duration of rupture of membranes, birth weight, or Apgar score. It was concluded that the mature human fetus produces gastric acidity in response to stresses associated with labor and vaginal delivery.