Prolactin is a protein hormone synthesized and secreted by the anterior pituitary gland. Because the monoamines dopamine and serotonin are important in the control of its secretion, prolactin has been the subject of much psychoendocrine research in recent years. The authors review some of the implications of the main findings of such research as they relate to schizophrenia, affective disorders, premenstrual syndrome, and alcoholism and discuss its possible usefulness to clinicians. As a research strategy, prolactin studies have a good potential for identifying specific neurotransmitters involved in discrete psychopathologic entities.