The yeast (Saccharomyces cerevisiae) pheromone response pathway is one of the best understood eukaryotic signal transduction pathways. Nonetheless, it is likely that components and regulators of the pathway remain to be identified. We have employed three approaches to learn about interactions among known pathway components and to identify new components. First, the two-hybrid system of Fields and Song revealed that STE5, a protein of unknown biochemical function, interacts with each member of the MAP kinase cascade. One interpretation of this finding is that STE5 facilitates interactions between members of the cascade and thereby makes signal transmission more efficient. Second, genetic studies have identified new gene functions that appear to be involved in pheromone response. One of these is homologous to RHO-GAP proteins, an observation that suggests that a RHO protein (members of the RAS super-family) is part of the response pathway. A second gene function, FAR3, appears to be required only for a specific facet of pheromone response, arrest of the mitotic cell division cycle in G1.