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1. The concentration of Ca in the arterial blood plasma perfusing submandibular glands of anaesthetized cats and possums (Trichosurus vulpecula) was altered by constant-rate close arterial injection of isotonic CaCl₂ or EDTA solutions.

2. There were no significant changes in the resting potentials of surface acinar cells in either species when the arterial plasma Ca concentration was altered over the range 0-30 mM.

3. Secretory potentials in response to parasympathetic stimuli and close arterial injection of ACh were reduced and finally abolished during EDTA injection, but temporarily restored by injection of CaCl₂.

4. Secretory potentials in response to 1 Hz parasympathetic stimuli in the cat were enhanced by increasing the gland arterial Ca concentration up to 5 mM; but responses to 10 Hz stimuli or close arterial ACh injection were not affected. There was no such enhancement of secretory potentials in the possum.

5. Increase in the gland arterial plasma Ca concentration above 5 mM caused reduction in the amplitudes of the secretory potentials to all stimuli in both species, until they were abolished at 7-10 mM in the possum and 15-20 mM in the cat.

6. In both species, increase in the gland arterial plasma Ca concentration caused increase in the gland blood flow and enhancement of the vasodilator effects of parasympathetic stimuli and ACh injection. EDTA injection was sometimes associated with a vasoconstrictor response to parasympathetic stimuli.

7. It is concluded that the secretory potentials, but not the resting potentials, of submandibular secretory cells require the presence of Ca in the interstitial fluid. It is suggested that Ca affects both the release of transmitter and at higher concentration the mechanism by which it activates the secretory cells.