For the past ten years, research in our laboratory has focused on the synaptic circuitry of GABAergic and glutamatergic networks in the monkey basal ganglia, a group of subcortical brain structures involved in the control of motor and psychoaffective behaviors. Elucidating the distribution and synaptic localization of different subtypes of glutamate and GABA receptors and transporters is essential to understand the basic functioning of these brain structures and develop better therapies for neurodegenerative diseases such as Parkinson's and Huntington's diseases. Although immunoperoxidase labeling allows to localize receptors at the cellular level, this method is generally not suitable for quantification and exact synaptic location of receptors due to the diffuse nature of the peroxidase reaction product. Here, we describe a highly sensitive post-embedding immunogold technique on Lowicryl-embedded ultrathin sections to analyze the subsynaptic distribution of these receptors at the electron microscope level.