After a surgically induced partial obstruction of the small intestine (ileum) in adult rats there is an accumulation of ingesta and a progressive enlargement of the lumen accompanied by wall thickening: over a period of 2–3 wk the circumference of the hypertrophic intestine increases by a factor of 2·7 and the thickness of the musculature increases more than threefold, while the length of the ileum (measured at the mesenteric attachment) remains unchanged. The villi become markedly larger and more elongated in the circumferential direction, and have a greater separation between one another. The number of villi per unit surface is markedly reduced but the number of villi per unit length of ileum, whilst appearing to show a small increase, was not significantly altered. The component epithelial cells (absorptive cells) appear unchanged in morphology and size (height). The microvilli of the epithelial cells have the same appearance, size (height) and packing density in the control and the hypertrophic ileum. Glands of Lieberkühn, Peyer's patches and single lymphatic follicles constituting the Peyer's patches are significantly increased in size in the hypertrophic intestine. The serosal surface of the hypertrophic ileum, in spite of the great expansion, remains regularly covered by mesothelial cells; these are much larger than in the controls and have an altered distribution of their microvilli.