Over the last two decades, the Drosophila larval neuromuscularjunction has gained immense popularity as a model system forthe study of synaptic development, function, and plasticity.With this model, it is easy to visualize synapses and manipulatethe system genetically with a high degree of temporal and spatialcontrol, which makes it ideal for resolving problems in synapticphysiology and development. This protocol describes an assaythat can be used for observing the cycling of transmembraneproteins at the plasma membrane and their trafficking withincells when there are antibodies available that bind cell-surfaceepitopes in vivo.