While prolactin is best known for its role in mammals in promoting milk production from mammary glands, vertebrates developed prolactin long before mammals evolved. Prolactin is known to have more than 300 functions in vertebrates, more than the roles of all other pituitary hormones combined. One of its major roles in fish is the control of osmotic balance (Manzon, 2002).

A number of tissues are known to produce (or at least contain) prolactin including the placenta, brain, GI tract, ovary, uterus, testes, prostate, islets of Langerhans, and parts of the immune system. Prolactin receptors are expressed in the olfactory system, adrenal cortex, skeletal system, liver, ovary, uterus, testis, epididymis, prostate, and some parts of the immune system and brain (Tillmann, 2002). Prolactin is involved in the formation of the corpus luteum and, in mice, its release after mating is required for the maintenance of the otherwise short-lived corpus luteum (although in mice elevated prolactin levels last much longer after mating) (Tillmann, 2002).

An increase in prolactin after orgasm seems to represent a signal for sexual satisfaction and the inhibition of arousal. This prolactin release does not occur without orgasm. The increase of prolactin 4 times greater after sexual intercourse compared to masturbation suggests that greater satiety is associated with intercourse (Brody, 2006a; Exton, 2001; Tillmann, 2002). Prolactin reduces libido and gonadal function in humans and sexual activity in animals and prolactin seems to be a factor in the male refractory period after ejaculation (Tillmann, 2002).

Hypersecretion of prolactin results in the loss of menstrual cycles and reduced sperm production (Tillmann, 2002). Some drugs, such as neuroleptics and antidepressants, can increase prolactin levels, decrease sexual drive, cause delayed orgasm in men and women, or anorgasmia in women (Tillmann, 2002). Hyperprolactinemia is associated with decreased libido and sexual disfunction. Some of the effects of chronic antidepressant and antipsychotic medications are through changes in prolactin release (Exton, 2001). Female sexual dysfunction occurs in almost 90% of women who suffer from hyperprolactinemia. Hyperprolactinemia is the most common endocrine disorder of the hypothalamic-pituitary axis (Kadioglu, 2005).

Cortisol, LH, FSH, GH, E, testosterone, and ß-endorphin are not affected by orgasm after masturbation. Levels of NE increase during orgasm briefly and prolactin levels remain elevated for a half hour afterwards (Kruger, 1998).