The *alpha*-factor pheromone receptor activates a G protein signaling cascade that stimulates yeast conjugation. In order to investigate the mechanism of receptor activation, we are analyzing constitutively active receptor mutants. Mutation of a proline residue in transmembrane domain 6 (TMD6) activated receptor signaling. Interestingly, TMD6 is directly linked to the intracellular sequences that are involved in G protein activation. The role of TMD6 was explored further by mutating polar residues in TMD6 to leucine. The results indicate that mutation of some of the polar residues caused constitutive receptor activity. These polar residues are likely to form intramolecular contacts that keep the receptor in the inactive conformation. The activated receptors must also be properly regulated in order to promote efficient mating. The receptor C terminus acts as a regulatory domain that promotes adaptation to *alpha*-factor. Our studies have shown that receptors are regulated by phosphorylation (Mol. Cell. Biol. 16: 247). Mutation of four serine and threonines in the distal C terminus to alanine decreased receptor phosphorylation in vivo and caused about 2.5-fold higher pheromone sensitivity. A genetic screen is being carried out to identify the kinase that phosphorylates the receptor. Nine candidates have been isolated as being more sensitive to *alpha*-factor. These mutants are not complemented by the known adaptation gene SST2 and are being studied for a role in receptor phosphorylation.