The alpha-subunit of anterior pituitary glycoprotein hormones is present in its free form both in normal men and in pre- and post-menopausal women. Serum levels may vary under the influence of hypothalamic hormones such as TRH or LHRH. The alpha-subunit originates in the pituitary gland where it can be found at an early stage of foetal life. Serum levels may rise substantially in several pathological conditions, including pituitary adenoma, and may respond to different factors. The alpha-subunit has also been demonstrated in rats and bovines. Recently, a role has been discovered for it in the rat: it seems to induce anterior pituitary lactotrope differentiation during foetal life. Its possible role in man during foetal life and during the development of pituitary adenoma is envisaged in relation to currently known data.