It has long been known that caffeine has dilator effect upon the renal vessels. There is, however, no decided theory as for the dilator action of caffeine. Sollmann, Pilcher and others concluded that the dilator action of caffeine upon the blood vessels is due to the paralysis of sympathetic nerves, which Stillnumkes, Desgrez and Dolean supporting, On the other hand, Yamamoto and some others, by their observation, are of another opinion that the dilator action of caffeine is based on the stimulation of the vasodilator nevres. I have attempted to decide the question and come to the following results. Caffeine causes the dilatation of renal vessels, when it is perfused, notwithstanding the fact that the sympathetic nerves are degenerated completely; it must act, therefore, upon "Myoneural junction" or muscles themselves. And even when the action of adrenalin upon the renal vessels was baffled perfectly by perfusion of quinine chloride, still caffeine produces appreciable dilator effect, which proves that caffeine operates upon muscles themselves.