1. During the uptake of chloride from an external choline chloride solution, electroneutrality appears to be preserved by excretion of base, but base excretion independent of chloride absorption also occurs.
2. Sodium ion uptake from sodium sulphate solutions is compensated by excretion of hydrogen ions, but this has been established only in animals adapted to distilled water.
3. In animals preadapted to running water, the sodium ion influx can be abolished almost totally by amiloride without diminishing ammonium excretion.
4. The net fluxes of sodium and chloride are inhibited by acetazolamide and cyanide.
5. These results are interpreted as indicating that Na+ and Cl- are absorbed chiefly in exchange for H+ and HCO3-, and are discussed in relation to an exchange diffusion mechanism.