1. The exchanges of sodium and bromide (for chloride) ions between the brackish-water prawn Palaemonetes varians and its environment are described.
2. In an isosmotic medium the exchange of sodium and chloride ions takes place by passive diffusion.
3. In full-strength sea water sodium ions are actively removed extrarenally, the potential difference produced by the active extrusion of sodium ions maintaining the chloride ions in passive equilibrium. There is some evidence of an increased flux of ions in hyperosmotic sea water associated with water-swallowing to obtain water for water balance.
4. In 2% sea water chloride ions are actively absorbed, the potential produced by this active uptake helping to maintain sodium balance; but some active sodium uptake also occurs.
5. In salinities below 2% uptake of ions declines and the animals can no longer maintain equilibrium.