Alternative names for prolactin (PRL) found in the older literature include mammotropin, lactogen, and luteotropin. One biological action of early prolactinrich extracts of adenohypophysis was to exert a luteotrophic action on the corpora lutea of rats and mice. Chronic hyperprolactinemia is often associated with suppressed levels of adenohypophysial gonadotropins, primarily luteinizing hormone (LH). In male and female rats, specific experimental conditions are necessary to demonstrate that hyperprolactinemia suppresses LH secretion. PRL is unusual among adenohypophysial hormones because it is present in milk at concentrations equivalent to those in maternal blood. PRL is the only adenohypophysial hormone that can be secreted in substantial quantities without significant hypophysiotrophic stimulation from the hypothalamus. Mammotroph cells of the pars anterior are subject to autocrine/paracrine regulation in addition to neurohormonal regulation by PRL-inhibiting hormone and PRL-releasing hormone compound. Evidence for PRL-induced increases in dalton neurosecretion by tuberoinfundibular neurons is very strong.