The sensitivity of different segments of the intestine to the stimulation of tartar emetic was markedly different. When the drug solution was introduced into the isolated fistula or duodenum or jejunum, both vomiting and diarrhea may be observed. When the same solution was introduced into the lower portion of ileum or colon of the same or other animals, the vomiting did not occur but the bowel moved alone. The aforementioned experiments also indicate that the gastrointestinal reaction was due to reflex. The estimation of the amount of absorption of tartar emetic in the different segments of the intestine (duodenum, jejunum,ileum and colon) is as follows: The amount of absorption of tartar emetic was measured and compared with the concentration of tartar emetic in plasma after directly introducing the tartar emetic solution into the different fistulas in conscious dogs. Further studies were made by directly measuring and comparing the amount of disappearance of drug from the intestinal loops in anesthetized rabbits. The amount of absorption of tartar emetic from the upper segment of intestine such as duodenum and jejunum was greater than that in the lower segment such as colon. The degree of absorption of tartar emetic in the different pH media has been studied by continuous perfusion of the whole length of the small intestine with drug solution in the anesthetized rats. The pH of the intestinal medium was an important factor in the absorption of tartar emetic by intestine. The amount of absorption of tartar emetic in the acidic medium (pH 5.7) was greater than at pH 7.2.