The functional coupling of heterologous G protein-coupled receptors (GPCRs) to the pheromone-response pathway of the budding yeast Saccharomyces cerevisiae is well established as an experimental system for ligand identification and for characterizing receptor pharmacology and signal transduction mechanisms. A number of groups have developed yeast strains using various modifications to this signaling pathway, especially manipulation of the G protein alpha subunit Gpa1p, to facilitate coupling of a wide range of mammalian GPCRs. The attraction of these systems is the simplicity and low cost of yeast cell culture enabling the assays to be set up rapidly in academic or industrial labs without the requirement for expensive technical equipment. Furthermore, haploid yeasts contain only a single GPCR capable of activating the pathway, which can be deleted and replaced with a mammalian GPCR providing a cell-based functional assay in a eukaryotic host free from endogenous responses. The yeast strains used for this purpose are highly engineered and may be covered by intellectual property for commercial applications in some countries. However, they can usually be obtained from the host labs for research purposes covered by a Material Transfer Agreement and/or licence where appropriate. The protocols herein assume that such strains have been acquired and begin with introduction of the heterologous GPCR into the engineered yeast cell. Assays are configured such that agonism of the GPCR leads to induction of a reporter gene and/or growth of the yeast. A number of parameters may be optimized to generate robust experimental formats, in high-density microtiter plates, that may be used for ligand identification and pharmacological characterization.