Little is known about the ontogeny of gastric acid secretion in the very preterm infant. In order to study this we recorded intragastric pH continuously for 24 h on 71 occasions in 22 enterally starved preterm infants. Infants ranged from 24 to 29 weeks' gestation and were studied in the first 5 days, and in the third week, of life. As the infants became more mature, both in terms of gestation and postnatal age, there was a decrease in intragastric pH from median (range) 3.7, 2.5 (0.6-3.9) and 1.8 (1.3-2.6) for infants of 24-25, 26-27 and 28-29 weeks' gestation, respectively on the first day of life to 1.8 (1.7-1.9), 2.0 (1.8-2.3) and 1.7 (1.5-2.0) on day 16. All the infants were able to maintain a gastric pH of below 4 from the first day of life. Our data lay to rest the suggestion that the preterm infant is incapable of hydrogen ion secretion. Gastric acid secretion in the newborn preterm infant should allow normal proteolytic activity and the well recognised clinical problems of intragastric bleeding, gastritis or oesophagitis may be attributable to intragastric acid.