Proteomics has evolved from genomic science due to the convergence of advances in protein chemistry, separations, mass spectroscopy, and peptide and protein databases. Where identifying protein-protein interactions was once limited to yeast two-hybrid analyses or empirical data, protein-protein interactions can now be examined in both cells and native tissues by precipitation of the protein complex of interest. Coupling this field to receptor pharmacology has recently allowed for the identification of proteins that differentially and selectively interact with receptors and are integral to their biological effects. It is becoming increasingly apparent that receptors in neurons do not exist as singular independent units, but rather are part of large macromolecular complexes of interacting proteins. It is a primary quest of neuroscience to piece together these interactions and to characterize the regulatory signalplexes of all proteins. This unit presents co-immunoprecipitation-coupled mass spectroscopy as one way of identifying signalplex partners.