The transport of D-glucose and L-phenylalanine was measured in intestinal brush border membrane (BBM) vesicles treated with Excretory-secretory (ES) products of Giardia lamblia. Uptake was found to be significantly lower (P/0.01) in the treated vesicles than in the controls. Exposure of intestinal tissue to ES products resulted in net secretion (P/0.01) of Na+, C1- and 3-O-methyl-D-glucose. Both observations indicate that alterations in the absorptive functions of the intestine might be attributed to interaction of ES products with the BBM.