1. Sodium influx in ammocoete larvae increases exponentially with external sodium concentration (0-1.0 mM/l.) and sodium-depleted animals show a 20% increase compared with normal animals.
2. Sodium loss decreases as the environmental concentration decreases, although the reverse situation is expected from considering diffusion outflux alone.
3. It is argued that part of the sodium loss is back-transported by the transport mechanism and this accounts for the reduced sodium loss from sodium-depleted animals whose sodium carrier activity is increased. The curves relating back-transport to environmental sodium differ from those derived by Kirschner for isolated frog skin.
4. Sodium influx increases as sodium loss increases, indicating a self mechanism whose features are discussed. In the ammocoete, the sodium carrier mechanism appears to change in affinity for sodium (short-term response) and can also change in concentration (long-term response), and it is suggested that these features, together with permeability changes, may form the basis of the controlling mechanism for sodium balance.