We report here electrophysiological and ion distribution results obtained with mechanically skinned skeletal muscle fibres which indicate that a Donnan equilibrium, characterized by large ionic concentration and electrical potential differences, is established between the myofibrillar space and the rest of the sarcoplasm surrounding the myofibrils. The more negative electrical potential within the myofibrils (-15 to -29 mV for conditions similar to those in vivo) can strongly influence: (1) intracellular Ca2+ movements associated with contraction; (2) the location of sarcoplasmic enzymes, their substrates and products of reaction; (3) measurements of the electrical potential difference between the extra- and intracellular space and estimation of relative membrane permeabilities for certain ions.