Publisher Summary The posterior pituitary contains a strong uterotonic activity known as the nonapeptide oxytocin (OT). It has been determined that uterus itself represents a major site of OT production. Therefore, the activation of uterine contractions by circulating OT is only one aspect of OT action. A significant portion of OT activity originates from within the uterus and, therfore, acts via autocrine and paracrine mechanisms. This chapter provides an overview of the role of circulating oxytocin and its receptors. The uterus contains its own intrinsic OT system. The ligand and its cognate receptor are both expressed in the same organ and, to some extent, in the same cells. The experiments described in the chapter showed that the genes for the receptor and its cognate ligand were subject to strict and very dramatic regulation during gestation and also, to a lesser extent, during the estrous cycle.