Mating in Saccharomyces cerevisiae is initiated by the secretion of diffusible peptide pheromones that are recognized by G protein-coupled receptors (GPCR). This review summarizes the use of the alpha-factor (WHWLQLKPGQPMY)--GPCR (Ste2p) interaction as a paradigm to understand the recognition between medium-sized peptide hormones and their cognate receptors. Studies over the past 15 years have indicated that the alpha-factor is bent around the center of the pheromone and that residues near the amine terminus play a central role in triggering signal transduction. The bend in the center appears not to be rigid and this flexibility is likely necessary for conformational changes that occur as the receptor switches from the inactive to active state. The results of synthetic, biological, biochemical, molecular biological, and biophysical analyses have led to a preliminary model for the structure of the peptide bound to its receptor. Antagonists for Ste2p have changes near the N-terminus of alpha-factor, and mutated forms of Ste2p were discovered that appear to favor binding of these antagonists relative to agonists. Many features of this yeast recognition system are relevant to and have counterparts in mammalian cells.