oxytocin (ŏksĭtōˈsĭn) [key], hormone released from the posterior lobe of the pituitary gland that facilitates uterine contractions and the milk-ejection reflex. The structure of oxytocin, a cyclic peptide consisting of nine amino acids, was determined in 1953 and in the same year it was synthesized in the laboratory. Both oxytocin and antidiuretic hormone are biosynthesized in the hypothalamus of the brain and travel down neuronal axons to the posterior pituitary, where they accumulate prior to release. Stimuli that elicit the release of oxytocin include childbirth, suckling, and coitus; the uterine contractions that result may facilitate either childbirth or the ascent of spermatozoa through the fallopian tubes. Oxytocin may also play a role in the initiation of labor. The milk-ejection response occurs only in females immediately after childbirth. The role of oxytocin in males is unknown.