1. Regulation of cell volume utilizing intracellular free amino acids has been studied in isolated ventricles from marine bivalves of the genus Modiolus.

2. As in the intact animal, ventricles taken from Modiolus acclimated to various salinities show only a slight change in tissue hydration. This control over cell volume is accomplished by isosmotic intracellular regulation of taurine, alanine, glycine and proline concentrations.

3. When stressed with decreased external salinities the isolated spontaneously beating ventricle becomes quiescent for a period, and then resumes activity. During the period of quiescence ninhydrin-positive substances (NPS) are released. The duration of quiscence and the amount of NPS released increase with increasing dilution of the external medium.

4. The salinity-induced NPS efflux is composed of taurine, alanine, glycine and proline.

5. In molluscs, the amino acids utilized for volume regulation are released from the cells unchanged and are not degraded into keto-acids and ammonia as they are in the crustaceans.

Contribution no. 10 from the Tallahassee, Sopchoppy and Gulf Coast Marine Biological Association