Rat jejunum was perfused for two and a half hours with aqueous ethanol (volume fraction 0.05) or aqueous carbon tetrachloride (volume fraction 0.005). Membrane vesicles subsequently prepared from the brush border displayed markedly decreased contents of cholesterol and phospholipids, as well as decreases in the activities of membrane-bound enzymes. These decreases were more pronounced in vesicles prepared after carbon tetrachloride than after ethanol perfusion. It is suggested that the anchoring of membrane-bound enzymes is decreased by physical chemical alterations in the hydrophobic region of the membrane.