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In isolated bundles of external intercostal muscle from normal goats and goats with hereditary myotonia the following were determined: concentrations and unidirectional fluxes of Na+, K+, and Cl-, extracellular volume, water content, fiber geometry, and core-conductor constants. No significant difference between the two groups of preparations was found with respect to distribution of fiber size, intracellular concentrations of Na+ or Cl-, fiber water, resting membrane potential, or overshoot of action potential. The intracellular Cl- concentration in both groups of preparations was 4 to 7 times that expected if Cl- were distributed passively between intracellular and extracellular water. The membrane permeability to K (PK) calculated from efflux data was (a) at 38°C, 0.365 x 10-6 cm sec-1 for normal and 0.492 x 10-6 for myotonic muscle, and (b) at 25°C, 0.219 x 10-6 for normal and 0.199 x 10-6 for myotonic muscle. From Cl- washout curves of normal muscle usually only three exponential functions could be extracted, but in every experiment with myotonic muscle there was an additional, intermediate component. From these data PPcl could be calculated; it was 0.413 x 10-6 cm sec-1 for myotonic fibers and was 0.815 x 10-6 cm sec-1 for normal fibers. The resting membrane resistance of myotonic fibers was 4 to 6 times greater than that of normal fibers.