Effects of monovalent cations and some anions on the electrical properties of the barnacle muscle fiber membrane were studied when the intraor extracellular concentrations of those ions were altered by longitudinal intracellular injection. The resting potential of the normal fiber decreases linearly with increase of logarithm of [K+] the internal K+ concentration is decreased the overshoot of the spike potential increases and the time course of the spike potential becomes more prolonged. In substituting for the internal K+, Na+ and sucrose affect the resting and spike potentials similarly. Some organic cations (guanidine, choline, tris, and TMA) behave like sucrose while some other organic cations (TEA, TPA, and TBA) have a specific effect and prolong the spike potential if they are applied intracellularly or extracellularly. In all cases the active membrane potential increases linearly with the logarithm of [Ca++]out/[K+]i n and the increment is about 29 mv for tenfold increase in this ratio. The fiber membrane is permeable to C1-and other smaller anions (Br-and I-) but not to acetate-and larger anions (citrate, sulfate-, and methanesulfonate-).

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