The range of activity and the location of lipase and pepsin were determined in the stomach and duodenum of infants, children, and adults. The range of lipase activity in biopsy specimens from the gastric body, in 29 subjects aged from 3 months to 26 years, was 1.8-5.3 U/mg protein (1 U is 1 mumol [3H]oleic acid released from tri-[3H]olein per minute). There were no significant differences among age groups (5-19 months, 2-4 years, 6-10 years, 11-13 years, and 15-26 years). Lipase activity was low or undetectable in the gastric antrum of all subjects. Pepsin activity in specimens from the gastric body ranged from 180 to 780 pepsin units/mg protein (using hemoglobin as substrate). The antrum had significantly lower pepsin activity (P less than 0.001) than the gastric body. As with lipase activity, there were no statistically significant differences in pepsin activity among age groups. Lipase and pepsin activity was also quantified in pinch biopsy specimens from the duodenum and duodenal bulb in 13 subjects. Contrary to lipase activity, which was almost completely absent from the duodenum or duodenal bulb, these sites contained low pepsin activity (9-78 pepsin units/mg protein). The data show that in infants and children, as previously reported in adults, gastric lipase is localized primarily in the gastric body. Tissue pepsin levels and localization, reported here for the first time, are similar to those of lipase, although, contrary to lipase, the gastric antrum has considerable pepsin activity. The identical levels of lipase and pepsin activities in infants, children, and adults indicate that the gastric phase of nutrient digestion is well developed at birth.