1. Addition of ATP to isolated highly purified Torpedo synaptic vesicles results in 45Ca2+ uptake. 2. Ca2+ dependent ACh release from Torpedo synaptosomes is accompanied by the phosphorylation of a specific protein with an apparent subunit molecular weight of about 100,000 (band alpha). 3. Activation of the presynaptic muscarinic receptors by an agonist inhibits Ca2+-dependent ACh release from Torpedo synaptosomes. This process seems to be mediated through an interference with the phosphorylation of the band alpha protein, and not by blocking the voltage-dependent presynaptic Ca2+ channel.