This study has assessed the contributions of the corticostriatal fibers, the ascending striatopetal fibers, and the intrinsic neostriatal neurons to the nerve terminal population found in the rat neostriatum (caudatoputamen). For this purpose, we have analysed the levels of two different synaptic vesicle-associated proteins, synapsin I and protein p38 (also called synaptophysin), in the neostriatum after specific lesions. Our results indicate that 45-50% of the synaptic vesicle proteins in the rat neostriatum derive from the corticostriatal fibers, that approximately 25-30% of the synaptic vesicle proteins are present in kainic acid-sensitive structures, presumably intrinsic terminals and local collaterals, and that ascending fibers contain 20-25% of the vesicle-associated proteins in the neostriatum. These three neuronal populations therefore comprise 95-100% of the synaptic vesicle-associated proteins in the rat neostriatum, and thus make up most of the nerve terminals in this brain region. The results, which are in general agreement with previous morphometric studies on the rat basal ganglia, therefore indicate that nerve terminals in the central nervous system can be quantitated by use of these biochemical nerve terminal markers. The results also indicate that a somewhat higher percentage of neostriatal nerve terminals belongs to the corticostriatal fibers that previously believed.