The small peptide hormone oxytocin has long been recognized as a key factor in the natural induction of the birth process at the end of pregnancy, as well as in the release of milk upon suckling by the infant. It has additional roles within the brain to modulate the control of the sodium balance as well as several maternal, affiliative and sexually related behaviors. Most oxytocin is synthesized within the large, magnocellular neurones of the hypothalamus from where it is transported down their axons to the posterior pituitary from which it is released in pulses upon appropriate neural stimulation. Oxytocin acts on the oxytocin receptor, which is a G-protein-coupled membrane receptor expressed at high levels in the uterus at the end of pregnancy, as well as on the muscle cells of the lactating breast. Especially within ruminants, such as cows and sheep, oxytocin and its receptor form part of a feedback loop regulating the control of the ovary, as well as providing a mechanism for the maternal recognition of pregnancy. Clinically, oxytocin has been used for many years to promote birth contractions. More recently, oxytocin-containing nasal sprays are being suggested as a way to influence certain kinds of behavior, though this is still experimental and controversial.