The present report describes the morphology and biochemical characteristics of two fractions containing rat striatal synaptosomes prepared by percoll density gradient centrifugation. The two fractions, designated fraction 3 and fraction 4, contained the same absolute number of synaptosomal profiles, however, fraction 4 contained a population of significantly larger synaptosomes. The mean diameter of synaptosomes in fraction 4 was 0.91 ?m, while synaptosomes in fraction 3 had a mean diameter of 0.59 ?m. Fraction 3 contained considerably fewer mitochondria, either within or external to the synaptosomes, and contained a greater number of synaptic plasma membranes than fraction 4. The larger fraction 4 synaptosomes contained less plasma membrane, but contained a larger percentage of mitochondria (10%). The distribution of binding sites for spiroperidol, SCH-23390, serotonin, muscimol, and nitrendipine, as well as adenylate cyclase and cAMP-dependent protein kinase activities were compared in membranes from these two fractions. In fraction 3 the number of recognition sites (B(max)) for D-1, 5-HT, GABA, nitrendipine, and forskolin receptors were 2- to 4-fold larger, while those for D-2 5-HT, adenylate cyclase and cAMP-dependent protein kinase activities were 1.5- to 2.0-fold greater than in fraction 4. In no case was there difference in K(d), suggesting that the number of binding sites was different. Since the radioligands that are preferential markers of receptor sites localized in postsynaptic membranes were concentrated in fraction 3, we conclude that this fraction may contain a large amount of postsynaptic membranes as well as the synaptosome component and may be suited to a variety of receptor binding studies. By contrast, fraction 4 synaptosomes contain presynaptic dopamine terminals [Robinson P.J. and Lovenberg W. (1986) Neurochem. Int.9, 455-458] and less nonsynaptosomal membrane contamination and should be valuable for functional studies on presynaptic mechanisms.