Experiments to compare the exchange (total influx) of sodium and chloride in the polychaete Nereis diversicolor in steady-state adaptation to very low salinities are reported. 2. The Na-uptake mechanism shows a high affinity for sodium, reaching half the maximal uptake rate at an external Na-concentration of 8-10 mM/liter (ca. 2% SW), and becomes "saturated" or reaches a plateau of uptake at concentrations of 40-50 mM/liter (ca. 10% SW) up to ca. 350 mM/liter (75% SW), above which Na-exchange is proportional to the external concentration. 3. The Cl-uptake curve differs from the Na-uptake curve in showing a relative depression at very low salinities before reaching "saturation" at Cl-concentrations of 50-60 mM/liter (ca. 10% SW). Cl-uptake becomes proportional to external concentration in salinities of 50% SW or greater, suggestive of passive diffusion in the ionic and osmotic conforming range. 4. It is shown that the permeability of the body wall, both to Na and to Cl, is reduced at very low salinities, thus destroying one of the assumptions upon which a previously-presented balance-sheet for chloride exchanges in N. diversicolor was based (Smith, 1970a). 5. Attempts to demonstrate an activation of the Na-uptake mechanism at very low salinities were inconclusive; reduction of body-wall permeability to sodium masks any possible activation. 6. It is suggested that the inside-negative body-wall potential is related to the depression of the Cl-uptake curve in salinities below 10% SW.
