Cells are provided with processes which maintain constant cell volume and electrolyte composition despite the difference in composition between cell and extracellular medium. The presence of charged fixed intracellular molecules generates a Donnan-type ionic distribution which, if unopposed, would induce continual entrance of extracellular water and permeant electrolytes. Constancy of cell electrolyte content and volume, under isosmotic conditions, depends on the activity of the Na⁺/K⁺ pump which compensates the cellward passive leak of sodium and the passive outward leak of potassium. As a result of the pump/leak interaction, potassium, and sodium ions are not absolutely free to traverse the cell membrane. A fraction of the potassium ions which leave the cell are reintroduced by the pump. Similarly, a fraction of the external sodium ions which enter the cells is reexpelled. Therefore, two operationally defined fractions of both ions are generated: one fraction is operationally immobilized by the pump and the other is free to diffuse and distribute according to the Donnan ratio imposed by the operationally immobilized fractions of sodium and potassium and the fixed intracellular ions. The osmotic behavior of stenohaline cells is a direct result of the constancy of cell osmolyte content: cell water content varies inversely with extracellular osmolality. Osmometric behavior is conserved despite acclimation to different osmolalities, substitution of external osmolytes and partial inhibition of the ionic pump by ouabain.