1. Molluscan cells maintain constant volume in decreasing salinities by releasing intracellular free amino acids as osmotic solute.

2. During a salinity decrease both osmotic pressure and ion concentrations decline simultaneously. Either condition might initiate the amino acid release.

3. Under constant osmotic conditions a decrease in Na+, K+ or Cl- concentration in the medium surrounding the isolated bivalve heart, did not result in an amino acid efflux.

4. On the other hand, removal of Ca2+ and Mg2+ resulted in a vigorous amino acid efflux which continued until the divalent ion concentration was restored.

5. The hypo-osmotically induced amino acid efflux could be partially blocked by raising the external divalent ion concentration. Similarly, lanthanum proved to be an effective blocking agent, both in low osmotic pressures and in Ca2+- and Mg2+-free SW.

6. These results show that salinity-induced free amino acid regulation is initiated by a decrease in external osmotic pressure. The time course of the efflux is dependent on external divalent ion concentration.

7. Finally, the site of divalent ion action is at the external membrane surface.

8. A hypothetical membrane model is proposed.

Contribution No. 18 from the Tallahassee, Sopchoppy and Gulf Coast Marine Biological Association.