Unlike other pituitary hormones, PRL is under tonic inhibition by the hypothalamus by way of the PRL inhibitory factor, dopamine. GAP and GABA may also be inhibitory. PRL-releasing factors include TRH and VIP and possibly others. Circulating PRL is predominantly monomeric, although some patients with hyperprolactinemia appear to have increased quantities of the less biologically active polymeric forms. PRL is secreted episodically, with an increase in the amplitude of the secretory pulses with sleep. A transitory increase also occurs in response to the protein component of meals. Basal PRL levels increase in response to the hormonal milieu of pregnancy, and suckling postpartum triggers PRL release. Pathologic increases of PRL owing to hypothalamic dysregulation occur with a variety of medications, including the neuroleptics, metoclopramide, antidepressants, methyldopa, reserpine and verapamil, abuse of opiates and cocaine, renal insufficiency, cirrhosis, hypothyroidism, adrenal insufficiency, neurogenic stimulation, and idiopathically. Hyperprolactinemia also may occur from structural lesions of the stalk and hypothalamus, which cause disinhibition with or without maintenance of PRF activity, ectopic neoplasm production, and, most commonly, from prolactinomas. Diagnostic testing consists of routine chemistry and thyroid testing and imaging with MRI or CT. Dopamine agonists are the treatment of choice of prolactinomas of all sizes. Transsphenoidal surgery is an alternative for the patient who does not respond to medical therapy or who wishes definitive tumor removal, realizing that long-term cure is achieved in only 50% to 60% of patients with microadenomas and a much lower number in those with macroadenomas. Radiotherapy is reserved for patients who do respond to either medical or surgical treatment. Patients wishing to become pregnant usually are treated with bromocriptine, although prepregnancy surgical debulking may be advisable for those with large macroadenomas to reduce problems with tumor enlargement.