We have attempted in a general way to test the hypothesis that a primary action of stimulants on cells is to cause an increase in the free Ca concentration within the cell. 1. The formation of crystals of calcium oxalate in the Elodea cell may be used as an intracellular method for detecting free Ca. 2. These crystals can be positively identified as calcium oxalate. 3. Condenser discharges and continuous direct currents cause the formation of crystals. The crystals are always formed at the anodal end of the cell. 4. Previous washing in solutions of oxalates and citrates prevents formation of these crystals. 5. Ultraviolet irradiation causes the formation of calcium oxalate crystals. Previous washing in oxalate prevents this. 6. Mechanical stimulation causes the formation of crystals. 7. Heat treatment within the temperature range 40°-55° causes the formation of crystals. Oxalate prevents this. 8. Plasmolysis followed by deplasmolysis causes the formation of crystals. Oxalate and citrate prevent it. 9. The effect of the oxalate and citrate may be reversed by subsequent immersion in Ca-containing solutions. 10. Isotonic salt solutions, and a series of toxic agents, do not cause the formation of crystals. 11. Anesthetics in non-lethal doses do not prevent the formation of crystals by electric currents or osmotic changes. 12. The results are shown to be consistent with the idea that when stimulants act on these cells, they cause a release of Ca from combinations located in the periphery of the cells.