Intracellular magnesium does not antagonize calcium-dependent acetylcholine secretion.

1. The effects of intracellular application of Ca and Mg ions on evoked acetylcholine secretion at frog motor nerve terminals were studied. Ca and Mg were applied to the nerve-ending cytoplasm using liposomes as a vehicle. 2. Under conditions in which intracellular application of Ca produced many-fold increased in evoked acetylcholine release. 3. When Mg was applied to the nerve-ending cytoplasm concurrently with Ca, acetylcholine release was further increased above the level produced by introducing Ca alone. 4. The results suggest that intracellular Mg does not antagonize depolarization-secretion coupling and that antagonism of transmitter release by extracellular Mg occurs only at the external surface of the nerve ending.