Present day concepts concerning the effects of tobacco smoking on gastrointestinal activities are based chiefly on clinical or personal experience. Many physicians can cite the history of one or more patients who were relieved of peptic ulcer, the ulcer syndrome or "unstable colon" when tobacco smoking was prohibited. However, little experimental data exist on which to base an explanation of the clinical observations referred to, and one is forced to reason a priori from the effects of various and frequently toxic doses of nicotine on animals and their isolated tissues or from the acute toxic manifestations experienced as a result of the "first smoke." We have studied systematically the effect of smoking from one to several cigarets on the various activities of the alimentary tract of normal human subjects, of patients with ulcer and of dogs. None of our subjects were required to smoke until the stage of acute toxic