AbstractNeurotransmitter release takes place by the exocytosis of loaded synaptic vesicles. The vesicles then fuse to the presynaptic membrane and are recycled by an endocytotic mechanism. A quantitative optical assay that detects uptake and release of a fluorescent dye during presynaptic activity was recently developed and used on the frog neauromuscular junction. I discuss a report(1) that demonstrates the effective application of this method to a Drosophila preparation. The authors use the shibire mutation and a spider venom to identify two intermediates in vesicle recycling. Their report, along with other recent studies, demonstrates the power and promise of the genetic approach for the understanding of mechanisms of synapse function and development.