Gastric pH was recorded with an intragastric pH electrode for 12 h in two groups of healthy, preterm infants with similar birth weights (range 1.4 to 2.0 kg). Group I infants (n = 13) were less than 7 days old and Group II infants (n = 10) were 7-15 days old. Infants were fed three formula feedings and one clear liquid feeding during the study. In Group I, mean gastric pH measured at 15-min intervals was above 4.0 for 3 h after either feeding. In Group II mean gastric pH was lower particularly after clear liquid feedings, where it remained below pH 4.0 for the entire 3-h postprandial period. The percent of monitored time at gastric pH less than 4.0 was low in Group I--15.2 +/- 4.2% and 20.6 +/- 6.4% after formula and clear liquid, respectively. The percent time was greater in Group II--42.7 +/- 8.0% and 61.9 +/- 7.3% after formula and clear liquid, respectively. In the younger preterm infant, gastric pH does not appear sufficiently low to support peptic activity.