Pentagastrin reduces the concentration of gastric mucosal histamine in the rat. This effect is counteracted by large doses of histamine H2-receptor agonists which are particularly effective if infused I.V. 2. After blockade of histamine H2-receptors but not of H1-receptors, the pentagastrin-induced reduction of mucosal histamine is greatly augmented and H2-receptor agonists no longer inhibit the response to pentagastrin. 3. It is suggested that in the rat stomach, H2-receptors occur on the histamine storing endocrine cells as well as on parietal cells and that mobilized mucosal histamine inhibits further release of histamine by way of H2-receptor-mediated auto feed-back.