The pH of gastric juice, obtained 3 to 4 minutes after birth in 158 unselected neonates, varied between 7.5 and 8.5 in 8 meconium-containing specimens and ranged from 1.4 to 7.8 in 150 meconium-free samples. In mature infants of the latter group, 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. In premature infants, pH was above 7 regardless of mode of delivery. Three was no correlation between neonatal gastric pH and pH of simultaneously obtained maternal gastric juice, pH of amniotic fluid, pH of cord blood 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. The possibility of a low gastric pH and the resultant pulmonary damage if aspirated must be considered in the initial care of thew newborn with poor muscle tone or reflex activity as well as in the anesthetic management of neonates undergoing emergency surgery.