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Morphometric and autoradiographic studies have been made of a crude synaptosomal preparation, which has been used extensively for membrane transport studies. When filters are used to separate membrane bound structures from incubation medium, the structures which survive filtration are those that are entrapped within the matrix of the filter structure. The population of membrane bound structures differs when one compares pellets of the preparation to sections of loaded 0.45 and 0.65 µm pore size filters. Both the relative numbers of synaptosomes, mitochondria, and other membrane bound structures (OMBS) and the mean size of each of the structures differ for pellet, 0.45 µm, and 0.65 µm filters. The percentage of total membrane bound volume attributable to synaptosomes increases from 28 in the crude preparation to 40 in 0.45 µm filters and 61 in 0.65 µm filters. The total volume of synaptosomes entrapped by differing pore size filters roughly correlates with the amount of substrate uptake. Neither mitochondrial volume nor the volume of other membrane bound structures was found to correlate with uptake. These results indicated that only the synaptosomes contribute measurably to this function. Autoradiographic studies confirm this conclusion. EM autoradiography following loading of the synaptosomal preparation with tritiated glutamate or GABA showed about 81% of the grains to be associated with synaptosomes. It is concluded that crude synaptosomal preparations may be used without further purification for membrane transport studies with unambiguous results.