The synaptic vesicle is the essential organelle of the synapse. Many approaches 
for studying synaptic vesicle recycling have been devised, one of which, the 
styryl (FM) dye, is well suited for this purpose. FM dyes reversibly stain, but 
do not permeate, membranes; hence they can specifically label membrane-bound 
organelles. Their quantum yield is drastically higher when bound to membranes 
than when in aqueous solution. This protocol describes the imaging of synaptic 
vesicle recycling by staining and destaining vesicles with FM dyes. Nerve 
terminals are stimulated (electrically or by depolarization with high K(+)) in 
the presence of dye, their vesicles are then allowed to recycle, and finally dye 
is washed from the chamber. In neuromuscular junction (NMJ) preparations, 
movements of the muscle must be inhibited if imaging during stimulation is 
desired (e.g., by application of curare, a potent acetylcholine receptor 
inhibitor). The main characteristics of FM dyes are also reviewed here, as are 
recent FM dye monitoring techniques that have been used to investigate the 
kinetics of synaptic vesicle fusion.