Oxytocin

Oxytocin is a nine amino acid peptide hormone synthesized in the hypothalamus and released by the posterior lobe of the pituitary gland.

Functions

Functions of oxytocin:

milk letdown -- in lactating mothers, oxytocin stimulates myoepithelial cells, causing milk to be ejected into the ducts of the mammary glands. Sucking by the infant at the nipple stimulates oxytocin release.
uterine contraction -- necessary for cervical dilatation prior to birth and causes the contractions all mothers are familiar with. Obstetricians often inject some synthetic oxytocin after the baby is born to speed the delivery of the placenta.
Inducing maternal behaviour -- Sheep and rat females given oxytocin antagonists after giving birth do not exhibit typical maternal behavior such as grooming towards newborns. Conversely, virgin sheep females show maternal behavior towards foreign lambs upon cerebrospinal fluid infusion of oxytocin, which they would not do otherwise.
Plays some role in orgasms -- for both males and females. In males, oxytocin is said to facilitate sperm transport in ejaculation.