The effects of lysolecithin, a normal constituent of duodenal juice, on gastric mucosa were measured under isolated conditions. In a relatively low concentration (0.5 mg/ml), lysolecithin, added to the luminal side, caused liberation of organic acids without altering the spontaneous rate of mineral (i.e., HCl) acid secretion by fundic mucosa. The low concentration of lysolecithin also did not appear to affect other active ion transport processes or permeability of either fundic or antral mucosa. At a higher concentration (1 mg/ml) lysolecithin inhibited spontaneous mineral acid secretion by fundus, altered active transport of other ions and increased mucosal permeability of both fundic and antral mucosa. Intraluminal lysolecithin in concentrations found in vivo may contribute to gastric mucosal damage.