A computer assisted morphometric method has been elaborated to quantify synaptic vesicles evidenced by means of the Zinc-Iodide-Osmium (ZIO) staining procedure in nerve endings of a very discrete area of the cerebellar granular layer: the glomerulus. The following parameters were calculated directly on electron microscopic negatives of 4.67 microns 2 of surface terminal area: number of vesicles per unit area (Na), and per unit volume (Nv), volume density (Vv), average diameter (d) and average volume of the single vesicle (V). Ultrastructural changes taking place at nerve endings also cover synaptic vesicles, thus quantitative studies regarding vesicle population at synaptic regions can be correlated to functional changes occurring in the process of chemical transmission and reflect the plasticity of synaptic junctional zones. Although this histochemical staining method generally is referred to as unspecific, after comparing our data with the available literature reports, we propose that ZIO-positive vesicles could have a physiological significance. These ZIO-positive organelles could take part in the intraterminal homeostatic control of Ca++ ions.