In dogs and cats, distention of esophagus, duodenum, jejunum or colon produced immediate inhibition of the empty or digesting stomach, as manifested by sharp and marked fall in tonus and cessation of contractions; inhibition of the cardiac and pyloric sphincters; and contraptions of the ileocolic sphincter. Distention of esophagus, stomach, duodenum, ileum, or colon produced similar responses of the jejunum, esophagus and colon. The inhibition phase is followed by a positive "rebound" phase, suggestive of that obtained after vagal stimulation of the heart. The inhibition appears to be fundamentally due to a rapid fall in tonus, rate and amplitude of contractions, varying proportionally with degree of tonus. Inhibitions of the stomach and intestine travel presumably over sympathetic efferent nerves and are not primarily due to any adrenal product, as the latent period is far less than the necessary circulation time. Distention of any portion of the gastrointestinal tract will produce inhibition of every other portion; the part stimulated will respond by contraction. In frogs and turtles it was found impossible to obtain long reflexes over the gut in either an orad or a caudad direction via the mechanisms resident in the gut walls, and in dogs and cats such reflexes could not be obtained via these mechanisms, at least in an orad direction. It is concluded, as a working hypothesis, that reflexes involving phylo-genetically old mechanisms in the gut act locally; and that reflexes acting in the gut at a distance from the stimuli are phylogenetically recent and involve recent nervous structures.