Several small (low molecular weight) GTP-binding proteins are associated with cholinergic synaptic vesicles derived from the electric organ of electric ray. Using GTP overlay techniques and direct micro sequencing we analyzed the association of small GTP-binding proteins with synaptic vesicles. Both experimental procedures revealed the specific occurrence of multiple small GTP-binding proteins with this organelle. Moreover, direct amino acid sequence analysis assigned at least three different small GTP-binding proteins, ora3, o-ral and o-rab3, to the vesicular compartment. Furthermore, the data reflect the relative abundance of these three proteins on the vesicle membrane, thereby demonstrating the predominant occurrence of o-rab3, the only exclusively synaptic vesicle specific small GTP-binding protein.GTP-binding protein; Cholinergic synaptic vesicle; Electric ray electric organ