The rectal pad of Schistocerca gregaria consists of a layer of large columnar epithelial cells and a layer of smaller oval-shaped cells. Both layers appear specialized for transport, as judged by the large number of mitochondria and membrane infoldings within the two cell types. The ultrastructure of the columnar epithelium and of the secondary cells is described as it appears under the electron microscope. The ability of the rectum to transport water and salts was tested in vitro. Unlike the in vivo preparation, the rectum in vitro does not transport potassium and chloride and has only a limited ability to transport sodium and water against a gradient. Dinitrophenol (10⁻³M.), iodoacetate (10⁻²M.) and ouabain (10⁻²M.) abolish water and sodium transport. Potassium cyanide (10⁻²M.) and ouabain (10⁻³M.) do not appear to inhibit water or sodium transport. Iodoacetate (10⁻³M.) inhibits sodium transport but does not affect water transport. The in vitro rectum is dependent upon anaerobic respiration. The results are discussed in terms of a scheme presented for in vivo cellular function. (Phillips, 1965). The studies of ultrastructure and transport physiology of the locust rectum do not refute the hypothetical schemes presented in this thesis. / Science, Faculty of / Zoology, Department of / Graduate