There is no morphologically detectable, continuous layer of extracellular mucus over the undamaged gastric mucosa of the rat. Instead, the mucosa is only partially covered by an interconnected network of mucous strands and sheets. This mucus is strongly acidic (sulphated) and is released by epithelial cells which line the isthmic and lower foveolar regions of the gastric glands of the fundus. A thick layer of gelatinous mucus is, however, released within 5 min after topical application of ulcerogenic agents. This mucus is released as a result of exfoliation and disintegration of interfoveolar surface epithelial cells. The released mucous glycoprotein is neutral or weakly acidic and is readily distinguished from the fibrous mucus produced by the epithelial cells which line the upper regions of the gastric glands. The preexisting network of acidic, fibrous mucus is retained at the luminal surface of the mucous cap which is produced over sites of damage. The layer of fibrous mucus is reinforced during the development of mucosal damage by accelerated release of mucous strands from the gastric glands. We propose that the fibrous mucus acts as an external, restraining layer to maintain the locally released, gelatinous mucus over sites of damage during a period in which epithelial continuity is restored by emigration of cells from the gastric glands.