1. The sodium content of the freshwater prawn Palaemonetes antennarius and the rates of influx and efflux of sodium in fresh water and a variety of other media are described.
2. The greater part of the influxes in all media is due to active uptake. The relationship between active uptake and the external and internal concentrations is described and it is concluded that the animal is living close to its viable limits of dilution.
3. The greater part of the efflux in fresh water is due to loss in the urine.