GABA and glutamate are known as the principal inhibitory and excitatory neurotransmitters in the vertebrate central nervous system, respectively. However, recent electro-physiological and immunogold literature data indicate that GABA may undergo also an excitatory action on presynaptic varicosities of parallel fibers (PFs) in the molecular layer of the rat cerebellum. PFs are axonal extensions, with a cross section of about 0.1 microm, of the glutamatergic granule cells. Such an unexpected excitatory action of GABA indicates clearly the presence of GABA receptors in the PFs of granule cells. We show in this study that quantum dots may be used specifically and efficiently to label two endogenous synaptic proteins, namely R-GABA(A)-alpha1 receptors (GABA(A) Rs) and glutamate transporters (VGLUT1) in order to target their localization in very small structures such as the presynaptic varicosities of the PFs.