Activity-dependent bulk endocytosis (ADBE) generates synaptic vesicles (SVs) during intense neuronal activity via a two-step process. First, bulk endosomes are formed direct from the plasma membrane from which SVs are then generated. SV generation from bulk endosomes requires the efflux of previously accumulated calcium and activation of the protein phosphatase calcineurin. However, it is still unknown how calcineurin mediates SV generation. We addressed this question using a series of acute interventions that decoupled the generation of SVs from bulk endosomes in rat primary neuronal culture. This was achieved by either disruption of protein-protein interactions via delivery of competitive peptides, or inhibition of enzyme activity by known inhibitors. SV generation was monitored using either a morphological horseradish peroxidase assay or an optical assay that monitors the replenishment of the reserve SV pool. We found that SV generation was inhibited by, 1) peptides that disrupt calcineurin interactions, 2) an inhibitor of dynamin I GTPase activity and 3) peptides that disrupt the phosphorylation-dependent dynamin I - syndapin I interaction. Peptides that disrupted syndapin I interactions with eps15 homology domain-containing proteins had no effect. This revealed that 1) calcineurin must be localised at bulk endosomes to mediate its effect, 2) dynamin GTPase activity is essential for SV fission and 3) the calcineurin-dependent interaction between dynamin I and syndapin I is essential for SV generation. We therefore propose that a calcineurin-dependent dephosphorylation cascade that requires both dynamin GTPase and syndapin I lipid deforming activity is essential for SV generation from bulk endosomes. This article is protected by copyright. All rights reserved.