Gastric acid secretion in response to meals of infant formula was studied in 8 healthy infants aged 4 to 24 weeks. The technique involved intragastric volume determinations using a marker and measurement of the acidity of the gastric contents. In the 2-hour postprandial period, pH declined from 7 to 5.2. The one-hour postprandial gastric acid output was 236 +/- 49 mumol/kg body weight.