Phosphorylation of synapsin I by CaMKII has been reported to mobilize synaptic vesicles from the reserve pool. In the present study, the distributions of alpha-CaMKII and of synapsin I were compared in synaptic boutons of unstimulated and stimulated hippocampal neurons in culture by immunogold electron microscopy. CaMKII and synapsin I are located in separate domains in presynaptic terminals of unstimulated neurons. Label for alpha -CaMKII typically surrounds synaptic vesicle clusters and is absent from the inside of the cluster in control synapses. In contrast, intense labeling for synapsin I is found within the vesicle clusters. Following 2 minutes of depolarization in high K(+), synaptic vesicles decluster and CaMKII label disperses and mingles with vesicles and synapsin I. These results indicate that, under resting conditions, CaMKII has limited access to the synapsin I in synaptic vesicle clusters. The peripheral distribution of CaMKII around vesicle clusters suggests that CaMKII-mediated declustering progresses from the periphery towards the center, with the depth of penetration into the synaptic vesicle cluster depending on the duration of CaMKII activation. Depolarization also promotes a significant increase in CaMKII immunolabel near the presynaptic active zone. Activity-induced redistribution of CaMKII leaves it in a position to facilitate phosphorylation of additional presynaptic proteins regulating neurotransmitter release.