Basal and pentagastrin-stimulated gastric acid secretion was measured in 26 healthy newborn infants on the 1st and 2nd days of life. Basal acid outputs (BAO) on day 1 (0.378 mEq/hr) and day 2 (0.388 mEq/hr) were similar and not significantly different from the maximal acid outputs (MAO) on day 1 (0.413 mEq/hr) and day 2 (0.452 mEq/hr). When calculated on a mEq/kg/hr basis, BAO on day 1 was 0.110 with an MAO of 0.122. The BAO on day 2 was 0.114 with an MAO of 0.133. Fasting serum gastrin levels obtained before the acid studies on each day were elevated (mean, day 1 = 100 pg/ml; mean, day 2 = 108 pg/ml). These findings suggest that either gastric acid secretion in the newborn is maximal under basal condition or that newborn parietal cells are unresponsive to pentagastrin on day 1 and 2 of life.