When maintained in organ culture, rabbit gastric mucosal biopsies incorporated [14tc]leucine into tissue protein and secreted labeled protein into culture medium steadily for 24 hr. Incorporation of radioactivity was abolished by cycloheximide. When examined by sodium dodecyl sulfate gel electrophoresis, dextran gel filtration, and ion exchange chromatography, 65 to 90% of macromolecular radioactivity secreted into culture medium migrated coincidentally with enzymatically assayed pepsinogen. Pepsin activity in cultured biopsies did not decrease during 24 hr of organ culture. Nevertheless, pepsin activity increased linearly in culture medium during this period. Acetylcholine markedly stimulated secretion of labeled protein and pepsinogen by cultured biopsies. In the presence of a subthreshold concentration (10(-10) M) of acetylcholine, pentagastrin, secretin, and the octapeptide of cholecystokinin, all stimulated protein secretion. Over-all incorporation of [14C]leucine into protein by cultured biopsies was stimulated by 10(-9) M pentagastrin. These results directly demonstrate: (1) synthesis and secretion of protein and pepsinogen by isolated gastric mucosa, (2) stimulation of gastric secretion of protein by acetylcholine and polypeptide hormones, and (3) stimulation of gastric synthesis of protein by pentagastrin.