Prolactin provides us with a window to the brain in our quest for understanding the psychobiology of depression, since the regulation of its release involves some of the monamine neurotransmitter systems that have been implicated in the pathophysiology of depression. Investigation examining basal prolactin plasma concentrations in depressed patients, including assessments of the rhythm of prolactin release, have not provided clear, consistent findings. Further exploration of the precise mechanisms involved in serotonin-stimulated prolactin release should shed light on the pathophysiology of abnormal prolactin responsivity in depression, and by extension, the psychobiologic basis of depression.