The pigeon crop-sac test is the oldest bioassay for prolactin. Prolactin stimulates the crop of pigeons and doves. Prolactin causes the crop-sac wall to thicken and the apical cells to desquamate. Prolactin is commonly measured by radioimmunoassay or by radioreceptor assay and numerous methods and antibodies for various species are available. Prolactin is secreted by acidophilic cells called lactotrophs or mammotrophs in the anterior pituitary gland. Prolactin is secreted from the pituitary gland in response to nursing, stress, mating, ovulation, estrogen treatment, and eating. The effects of prolactin in mammals are exerted primarily on the organs of reproduction. Membrane-bound receptor molecules for prolactin have been demonstrated in mammals in a variety of tissues, including liver, kidney, adrenal, ovary, testes, uterus, prostate, seminal vesicle, mammary gland, and tumors. The identity of the prolactin-releasing hormone, if one exists, and a complete uderstanding of paracrine and autocrine regulation of prolactin secretion within the pituitary gland need to be established.