The effect of acute schistosomiasis on the structure and function of the mouse small intestine has been examined. Whole loops of small intestine from normal mice, and mice seven weeks post-infection were incubated in an in vitro perfusion apparatus. The transport of glucose, 3-0-methylglucose, sorbitol and fluid were all markedly impaired in loops from the infected animals. Kinetic analysis of the data for glucose transport indicated that schistosome infection brought about a reduction in the total number, but not the affinity, of the absorptive sites for glucose. Under the scanning electron microscope, villi from normal mice were tall and erect, with intact surfaces devoid of mucus, whereas the villi from the infected animals were partially covered with strands of mucus and appeared swollen and eroded, with lesions around their apices.