1. Complete mechanical occlusion of the ileum was produced in dogs and the loops above and below the obstruction were examined functionally and morphologically 4 or 7 days later.
2. The intraluminal pressure in the occluded loop never exceeded 8 cm water.
3. The mucosa above the obstruction secreted water and ions into the lumen in vivo, though it absorbed glucose normally. Mucosal slices also absorbed amino acids and monosaccharides normally in vitro.
4. The mucosa below the obstruction absorbed ions and glucose in vivo and non-electrolytes in vitro to a slightly smaller extent than normal intestine.
5. The morphological changes above the occlusion included shorter, plumper villi and shorter crypts, a reduction in histochemically stainable brush-border enzymes, but an increase in acid phosphatase. Below the obstruction, there was atrophy of the villi and crypts and reductions in all enzymes studied.
6. The results suggest that the mucosa above the occlusion possesses an intact and almost normal epithelial layer, but that it has been stimulated to secrete in vivo, presumably by intraluminal factors. Below the obstruction, true atrophy of the mucosa has promptly developed.