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The book deals with a part of modern physiologic research in which progress made during the past sixteen years is conspicuous. In 1921 Otto Loewi reported his classic experiments, in which he showed for the frog's heart that stimulation of autonomic nerve fibers leads to liberation of definite chemical substances which influence the effector organs. Later, Dale, Feldberg, Cannon, Rosenblueth and many others not only proved the validity of this concept of chemical mediators of nerve action for the autonomic nerves of cold-blooded and warm-blooded animals but showed that acetylcholine is liberated at the neuromuscular junction of somatic nerve fibers and at the synapse in the superior cervical ganglion. The paradox that sweat glands are innervated by sympathetic fibers but react strongly to parasympathetic drugs was better understood when it was found that the sympathetic sweat fibers are "cholinergic" rather than "adrenergic." As to the nature of the adrenergic substance,