Cimetidine, which strongly stimulates prolactin release in healthy man, was unable to modify prolactin levels in prolactin-secreting pituitary adenomas. Furthermore, the infusion of 5-methylhistamine, an H2-receptor selective agonist, was equally ineffective. The results of these experiments together with the data reported in the literature suggest that H2-receptors modulate prolactin secretion at a hypothalamic level
