The regional and cellular distribution of four synaptic vesicle‐associated proteins, synapsins Ia and Ib (M_r 86,000 and 80,000, collectively referred to as synapsin I) and proteins IIIa and IIIb (M_r 74,000 and 55,000, collectively referred to as protein III), has been compared in selected rat brain regions, using both radioimmunoassays and back‐phosphorylation assays. Lesions of several neuronal populations in the basal ganglia (corticostriatal fibers, intrinsic striatal neurons, striatonigral fibers, nigrostriatal fibers) induced decreases in the levels of these various proteins that were highly correlated (r= 0.96–0.97). Moreover, the synaptic vesicle‐associated phosphoproteins displayed a similar and widespread distribution throughout the CNS. This evidence for colocalization indicates that the majority of, and possibly all, CNS neurons and nerve terminals may contain both forms of synapsin I and both forms of protein III.