In the orthodromic per-fusion of the denervated colonic loop of dogs with the normal saline solution, the preexisting waves, antidromic waves, are reinforced with a rise of pressure within the range from 0 to 12 cm H2O, however, without any outflow of fluid. When the pressure is raised higher than the level just mentioned, the orthodromic waves originate in the oral end of the loop, becoming more and more powerful with increase of the pressure and thus resulting in a remarkable outflow of fluid. Provided HC1 solution is used as the perfusing fluid, the events described above not only proceed more rapidly in succession but also occur more vigorously than those observed in the loop perfused with the saline solution. On the contrary, this succession of events does not occur where the mucosa previously has been cocainized. In the anti-dromic perfusion the outflow of fluid does not occur, because the enormous rise of tone, which is especially remarkable in the oral half of the loop, makes an extreme narrowing of the lumen of the loop. However, this phenomenon does not appear, provided the mucosa has been cocainized previously. It may be concluded that the mucosal intrinsic reflex regulates the strength and direction of contraction waves of the colon: The reflex reinforces the preexisting antidromic waves under a lower intraluminal pressure, favoring the contents to accumulate, whereas it elicits the orthodromic waves under a higher pressure, favoring the contents to be propelled anally.