The sodium, potassium, chloride, and water concentrations have been analyzed in relatively pure smooth muscles of electrophysiological interest and compared to similar analyses of other muscle types. The smooth muscles analyzed included isolated longitudinal myometrium from the uteri of various species and the taenia coli from guinea pigs and rabbits. The lack of accurate measurements of extracellular fluid volume was stressed and various assumptions were made to provide limiting values for this volume. Within these limits, various distributions of the ions studied were calculated. The outstanding feature of the ion distribution was the occurrence of an apparently hyperosmotic concentration of monovalent cation (Na + K) irrespective of its assumed volume of distribution. Evidence was discussed suggesting that this finding should be attributed to cation binding and/or large quantities of polyvalent intracellular anion. Evidence was presented suggesting active potassium uptake and active extrusion of sodium as the mechanisms of distribution of these ions. Chloride distribution was compatible with the assumption of distribution according to its equilibrium potential.