The gastric mucosa consists of the epithelium, which lines the lumen, the lamina propria, and the muscularis mucosae. The targets of drugs used to treat stomach and duodenal ulcers are thought to be the acid-secreting parietal cells of the epithelium. However, immune cells in the lamina propria are the only cells that showed detectable messenger RNAs for histamine, muscarinic, gastrin, and dopamine receptors by in situ hybridization histochemistry. None of the epithelial cells expressed any of these messenger RNAs. Thus, the targets of antiulcer drugs seem to be cells of the immune system in the gut and not parietal cells, as generally believed. This conclusion may revise the thinking about ulcer formation and may shed light on the etiology of such chronic small intestinal diseases as Crohn's disease.