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Crustacean muscle has been of special interest to physiologists because its myoneural junctions show the properties of central nervous synapses. The muscles are supplied by excitatory and inhibitory nerve fibres which can be stimulated separately, and whose interaction can be studied at the level of the nerve-muscle junction. As was pointed out in recent reviews (Wiersma, 1941;Katz, 1949), much valuable information has been obtained about the organization of the crustacean nerve-muscle system, but little progress has been made in elucidating the mechanisms by which antagonistic nerve impulses exert their effects on the muscle fibres.As a first step, it seemed necessary to study the properties of crustacean muscle fibres, quite apart from their nerve junctions. Hitherto, most of the elementary properties of the crustacean muscle membrane have remained unexplored, largely because of technical difficulties in obtaining suitable fibre preparations. With the help of intracellular electrodes, this difficulty can be overcome, and in the present work extensive use has been made of this method, for stimulating and recording across the surface membrane of individual muscle fibres.The object of this paper is to present electrical measurements on the crustacean muscle membrane, in particular its resistance and capacity, its resting and action potential, and its electrical reactions in various ionic environments. As was indicated in previous notes (Fatt & Katz, 1951b, 1952, the electric response of crustacean muscle fibres, and especially its persistence in sodium-free media, differs from that of many other excitable tissues, and the evidence for this will be discussed in detail. METHODSApplication of intracelular electrodes. The usual procedure was to introduce two microelectrodes into the same fibre, one to paas current through the membrane, the other to record the membrane potential. The circuits were completed by separate external electrodes whose connexions are shown in Fig. 1. The micro-electrodes were operated by separate manipulators and

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