Synapse formation can be promoted by intense activity. At the Drosophila larval neuromuscular junction (NMJ), new synaptic boutons can grow acutely in response to patterned stimulation. We combined confocal imaging with electron microscopy and tomography to investigate the initial stages of growth and differentiation of new presynaptic boutons at the Drosophila NMJ. We found that the new boutons can form rapidly in intact larva in Significance Statement Neurons can grow and form new synapses in response to intense activity. We investigated the stages of synapse formation at intact and dissected Drosophila larvae and identified a very rapid initial step, which is especially sensitive to nerve stimulation. Specifically, we demonstrated that intense activity triggers budding of new synaptic boutons filled with vesicles, and this pathway becomes very prominent under the conditions of pathologic activity, such as seizures. We found that this pathway depends on protein kinase A and its target synapsin, the protein regulating clustering of synaptic vesicles. These findings suggest a new function for dynamic vesicle clustering in neuronal development and demonstrate that this mechanism can create a positive feedback loop during seizure activity. New Research July/August 2019, 6(4) ENEURO.0060-19.2019 1-15 response to intense crawling activity, and we observed two different patterns of bouton formation and maturation. The first pathway involves the growth of filopodia followed by a formation of boutons that are initially devoid of synaptic vesicles (SVs) but filled with filamentous matrix. The second pathway involves rapid budding of synaptic boutons packed with SVs, and these more mature boutons are sometimes capable of exocytosis/endocytosis. We demonstrated that intense activity predominantly promotes the second pathway, i.e., budding of more mature boutons filled with SVs. We also showed that this pathway depends on synapsin (Syn), a neuronal protein which reversibly associates with SVs and mediates their clustering via a protein kinase A (PKA)-dependent mechanism. Finally, we took advantage of the temperature-sensitive mutant sei to demonstrate that seizure activity can promote very rapid budding of new boutons filled with SVs, and this process occurs at scale of minutes. Altogether, these results demonstrate that intense activity acutely and selectively promotes rapid budding of new relatively mature presynaptic boutons filled with SVs, and that this process is regulated via a PKA/Syn-dependent pathway.