Radsma and Tjhan have recently reported that the influence of adrenaline upon the methylenblue reducing action of frog's muscle was reversed at certain pH, Under the supposition that blood vessels are dilated or constricted through the action of adrenaline may be due to the influence of pH of the tissues, the author measured the pH of some organs of rabbits and investigated at the same time on the relation between the effect of adrenaline to the methylenblue reducing action of frog's muscle under several value of pH. Assumption was not fully realised, but an unexpected new fact was obtained, which may be described as follows: 1) Adrenaline accelerated the methylenblue reducing action of frog's muscle at about pH 7.4, but on either side of this pH the action of adrenaline was diminished gradually, until it was reversed at pH 6.1 or 9.0. 2) The extracts of cardiac muscle, skeletal muscle and lung of the rabbit with 1% common salt solution show similarily faint acid reaction, lying between pH 6.0-6.9. pH lues may be classified as follows: Cardiac muscle
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