Not much is known about the mobility of synaptic vesicles inside small synapses of the central nervous system, reflecting a lack of methods for visualizing these dynamics. We adapted confocal spot detection with fluctuation analysis to monitor the mobility of fluorescently labeled synaptic vesicles inside individual boutons of cultured hippocampal neurons. Using Monte Carlo simulations we were able to propose a simple quantitative model that can describe vesicle mobility in small hippocampal boutons under resting conditions and different pharmacological treatments. We find that vesicle mobility in a time window of 20 s can be well described by caged diffusion (D approximately 5 x 10(-5) microm(2)/s, cage sizes of approximately 50 nm). Mobility can be upregulated by phosphatase blockage and increased further by actin disruption in a dose-dependent manner. Inhibition of the myosin light chain kinase slows down vesicle mobility 10-fold, whereas other kinases like protein kinase C (PKC), A (PKA), and calmodulin kinase II (caMKII) do not affect mobility in unstimulated boutons.