It is well established that infection with Giardia lamblia can cause malabsorption although the pathogenesis of this is unknown. Transport studies were made on the intestinal segments of male albino rats infected with Giardia cysts obtained from human stools to investigate this pathogenesis. The results were compared with those seen in a normal group and in a control group that was fed with a Giardia-free stool suspension. There was a significant fall in the transport of glucose and glycine, both of which were actively absorbed. On the other hand, potassium, which diffuses passively in a bidirectional manner across the bowel wall, was not affected. Furthermore, it was observed that the transport mechanism was deranged in the presence of an entirely normal small intestinal mucosa, as judged by light microscopy. It is concluded that Giardia trophozoites interfere with the active transport mechanisms of the small intestinal mucosa. This defect could result either from structural damage to the substrate carriers or from an alteration in cell maturation.