Cation-exchange resins have not fulfilled early expectations. Those in clinical use do not retain much sodium in the feces when dietary sodium is much restricted. Comparison of the sodium and potassium bound by a resin, in equilibrium with presumably normal electrolytes as found in the terminal ileum, with the amounts of those ions in the feces, permits an over-all evaluation of two major factors operating in the gastrointestinal tract. These are a decreased sodium concentration in the terminal ileal contents when sodium intake is restricted and an absorptive power of the colon sufficient to detach sodium from the resin.