Localization of receptors in brain sections using autoradtographlc detection of radioligand binding has been an important technique in the neuroanatomical identification of a large number of neurotransmitter receptors. However, receptor autoradiography provides little information regarding the functional relevance of these sites and, in fact, does not establish which of the labeled receptor sites are actually coupled to intracellular signaling mechanisms. Fortunately, with the family of G protein-coupled receptors (GPCR), signal transduction is mediated at the level of the transducer itself (i.e., at the point at which receptors activate the α-subunits of G proteins to bind guanosine 5′-triphosphate [GTP]). The development of an assay for agonist-stimulated [35 S]guanylyl-5′-O-(γ-thio)-triphosphate ([35 S]GTPγS) binding, originally developed for receptors in isolated membranes, has provided an excellent opportunity to apply this process to brain sections, thus allowing the visualization of receptor-activated G proteins in specific brain regions.