Until a few years ago there existed a sharp dichotomy of opinion between the pharmacologists, who, on the basis of their experiments, assigned to acetylcholine the role of universal synaptic transmitter, and the electrophysiologists, who denied that this substance could effect the transfer of excitation from one conducting element to another. It is, however, now generally agreed that, certainly at the neuromuscular junction, and probably also at ganglionic and some—but not all—central synapses, acetylcholine does indeed exert a primary transmitter action. It must be added that recognition of this fundamental action has been hastened as a result of recent experiments by those very electrophysiologists (such as Eccles and his colleagues) who previously supported exclusively electrical hypotheses of nervous transmission. For this reason the assumption which is implicit throughout this paper that the acetylcholine in brain is concerned in maintaining central synaptic transmission is likely to go unchallenged and requires no detailed justification here.