Passive intestinal permeability has been investigated in rats infected by the nematode, Nippostrongylus brasiliensis, by the simultaneous administration of two probe molecules. Experiments using a closed intestinal loop show that there is a significantly increased absorption of lactulose and decreased absorption of mannitol in rats at the 10th to 11th day of infection. Experiments using serial oral administration techniques show that these changes start during the second week of the infection with a return towards normal values by the end of the third week. The results are similar to those found in human coeliac disease and add weight to previous studies which have stressed the similarity in the two disease processes.