Basal acid output and meal-stimulated acid output were measured in newborn infants after nasogastric infusion of 5% glucose and elemental formula, respectively. In six older infants (age range 6 to 31 months), basal acid output was 0.067 +/- 0.017 mmol/kg/hr and maximal acid output was 0.200 +/- 0.028 mmol/kg/hr. Meal-stimulated acid output in four of six older infants was 0.149 +/- 0.038 mmol/kg/hr. In eight healthy newborn infants basal acid output was 0.038 +/- 0.008 mmol/kg/hr; meal-stimulated acid output was 0.064 +/- 0.011 mmol/kg/hr (P less than 0.01). The time course of the secretory response to the elemental formula was as described previously after a protein meal in adults: the rate of acid secretion increased after 20 minutes and remained greater than the basal rate through the remainder of the 90 minute test period. These results demonstrate that in human newborn infants a mixed meal containing protein hydrolysate induces an acid secretory response that is qualitatively similar to but weaker than the response in older infants and adults.